Power Platform Community Forum Thread Details

I am just posting this on how to have a Shared Mailbox show in Power Automate drop down. And also the additional way to get the Calendar ID of a shared mailbox, if you still want to do this for other reasons.
Prerequisites:

You need to be an owner of the shared mailbox related to the calendar.
You need to have the right Power Automate licensing to run the O365 Outlook connector under your own username.

Overview:
There are two main aspects to achieving this. However, after doing the first section, you’ll likely find that the shared calendar is now in the drop-down menu for your O365 Outlook Event triggers now anyway.
1. Setting yourself as the editor on a shared calendar – this will make the mailbox appear in your Power Automate drop down list.
2. Optional: Running a Get Calendar power automate to see the ID

Section 1 - Setting yourself as an editor on the shared calendar
Next, you need to set your permissions for the calendar against your own account as "can edit". Here are the steps:

In the browser, paste the following address (replacing the email address with your shared mailbox address):

https://outlook.office.com/mail/sharedmailbox@yourdomain.com/

The Outlook Web Mail will appear for this mailbox. Click the calendar icon on the left menu:

In the My calendars menu, click the ellipse next to the Calendar and select the Sharing and permissions option from the menu:

In the Sharing and permissions pop-up, you should see your user. Click the drop-down next to your settings and select Can edit from the menu. If your name isn’t appearing there, you may need to type it in the “Enter an email address or contact name field above” and Share:

In your personal inbox, you will now have an approval email that you need to Accept before this will work:

This will now mean that the calendar is attached to your username and if your account is the one attached to the Power Automate flow, then this will allow the calendar to show as if you were the owner. You don’t need to do the next step; the calendar will automatically show now! But I’ve included the next section just in case you need the ID itself.

Section 2 – Running the Get Calendar flow to obtain the Calendar ID of the Shared Calendar
Now that the calendar is attached to you, you can run a simple flow to obtain the Calendar ID.

In make.powerautomate.com, select My Flows in the left-hand menu, select New Flow drop-down menu from the top of the Flows screen and select Instant cloud flow from the menu:

The Build an instant cloud flow pop-up window will appear. Type in whatever flow name suits you (I’ve called this “Get Calendar ID”). Select the trigger called Manually trigger a flow. Click the Create button to open the flow box.

Click the + icon and select Add an action from the menu that pops up (it’s probably the only option):

In the Add an action pop-up window, type Get Calendar into the search field at the top of the screen and then select Get Calendars (V2) from the Office 365 Outlook action list:

At this point, it will either use an existing connection you have setup, or it will go through logging in to your account to connect to the O365 connector.
Under the Get Calendar action, click + and again select Add an action:

In the Add an action pop-up, type compose into the search bar and select the Compose action from the Data Operations group:

In the Compose pop-up, select the lightening bolt icon from the right-hand side of the inputs field:

And from the Get Calendar (V2) list, select the value Name first. This will have the value Name appear in the Inputs. Then type a <space>:

Click the lightening bolt icon again, and select value ID:

Your Compose input should look like this:

This will automatically add a “For Each” container to the Compose statement so you can obtain multiple results for all the calendars linked to your account. Your flow should look like this. Click the Save button to save the flow:

Now Run a Test by clicking the Test button next to the Save button:

Select Manually as the Test Flow option and click Test:

It may ask you to sign in to Office 365 Outlook connector as part of running the flow. Click Continue:

Then click the Run flow button:

Click Done to exit the Test pop-up window:

Your test will run and will likely be quick as most people don’t have many calendars attached. In this example, there are 4 calendars attached to this user. Click the Compose box in the flow, and it will open up the information that has been collected which includes the Calendar ID. Check the Calendar Name to make sure you have the right ID. In this case, the Birthday calendar came up first, which is not the one we are looking for. Therefore, click the right-hand arrow near the record ID to move to the next one and next one until you find the right calendar name:

Given I’ve just linked this calendar, the new calendar ID is actually the last one in the list. Click the copy button in the Outputs box:

Find a location that you can paste the output. For example, open Notepad, and paste the output in there. The Calendar ID is all the letters occurring after the Calendar name and space (note: I’ve scrambled this Calendar ID, so the underscores might not be in the normal locations):

To test this goes to the right calendar, open a browser and type https://outlook.office.com/calendar/ and then cut and paste the Calendar ID at the end of this. It should take you to the shared calendar if it is the right ID: