Hi, @Anonymous, and welcome.
You can get the ID/GUID of anything (lists, libraries, views, etc.) by going to it's Settings and checking the URL.
For example, if I select a list from my Site Contents and go to "List Settings" (click on the gear in the upper-right corner), you can find the list's ID in the URL that appears. For example: https://mycompany.sharepoint.com/teams/mySite/Outsourcing/_layouts/15/listedit.aspx?List=%7Bb91078c8-03ae-4289-be1d-2ecd29b3167d%7D
The ID itself is found after "List=". Some menus--and some browsers--display it differently. For example, it may appear encoded as "List=%7Bb91078c8%2D03ae%2D4289%2Dbe1d%2D2ecd29b3167d%7D" or decoded, as "List={B91078C8-03AE-4289-BE1D-2ECD29B3167D}" or even in-between, as "List=b91078c8%2D03ae%2D4289%2Dbe1d%2D2ecd29b3167d".
If it is encoded, you can certainly convert it manually (once you remember things like "%2D" is a hyphen, "%7B" is an opening curly brace, "%7D" is a closing curly brace, etc.). Or, you can use free online utilities like https://www.urlencoder.org/.
Generally, when providing an ID, you include the hyphens but not the surrounding curly braces. Thus, when prompted for an ID, the acceptable format is "B91078C8-03AE-4289-BE1D-2ECD29B3167D" (decoded, but without the curly braces.)
Note that there are some connectors where using the ID in place of selecting from the drop-down list of SharePoint Lists is a necessary "workaround." For example, the Get Items action displays your site's lists but not calendars. In these instances, selecting "Custom value" and then providing the item's ID is the only recourse.
Good luck... I hope this helps!