Hello,
First, you need a datasource to store your email sends requests, you can use a sharepoint list like this one :

Then, create a Power Automate Automated cloud flow and select "When an item is created" trigger :

Then,
- configure your "when an item is created" trigger with your SP site / list
- add an action "send an email V2" (outlook)
- optionnaly, delete the temp sharpeoint item

For sending email from a shared mailbox, use "from (send as)" in advanced options:

In your application, in the onselect off your send email control, patch your list with a new record :
Patch(TempMailSend,defaults(TempMailSend),{Title: Text(Now(),Shortdate),From: user().Email, To: TextInputRecipient.Text,Subject: TextInputSubject.Text, Body: TextInputBody.Text})
The mail will be send from the Flow's owner email (or shared mailbox set in "send as")
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