There are a lot of posts on line with code to find out a user’s GUID. Generally they are all variations on the WhoAmIRequest.
But what if you are non-coder and need the GUID to use in Microsoft Flow?
Here is the trick.
Go to the Settings, Security, Users Grid view.
Export to Excel.
Unhide the first 3 columns.
Take a look in the first column (Column A) which is the User GUID. Copy it and use it where you need it.