Before using the action “Send an email from the shared mail box”, have you created Shared mailboxes under Office 365 Admin center?
If yes, you would get an email address for the Shared mailbox. Then when using the action, you need to enter email address of the shared mailbox into the Mailbox Address field.
When the flow is triggered, an email would be sent out from the shared mailbox but not your personal mailbox.
Best regards, Mabel Mao
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