The first time you run a script written in Google Apps Script that requires
authorization in Google Sheets you'll see this 'Authorization Required' message:

A new window will open asking you to select an account
that you're currently logged in to (or choose another):

If you do know the author of the script and trust them (or you wrote it yourself)
you can proceed by clicking on the Advanced link in the bottom left corner:

This will open another section at the bottom of the window
that allows you to click on a Go to Script Name (unsafe) link.

This leads to the final window that allows you to grant the required permissions: