As an alternative solution, you could consider take a try to create a Guest account from your Organization for the external user, and then share the corresponding app through PowerApps to this Guest account within your Organization.
Please check and see if the following thread would help in your scenario:
I want to share powerapp with user in organization that do not have office365/powerapp subscription. They were able to access the Survey created using Microsoft forms. They signed in using there credentials(orgnizations internal email service credential) to complete the survey.
What to do the same thing for PowerApp data entry form.