I left more detail in ritchierich's app thread. But basically it asks for either a Gsuite account, or to send your app to be verified by Google and (by all appearances) pay a fee.
Does yours not use that same method? It appears to.
This is what I get when I choose external project.
In this mode, your app is available to any user with a Google account. External apps that request sensitive or restricted user data must first be verified by Google. This process may take 4-6 weeks. In addition, if you selected restricted scopes, you'll need to go through an independent security review, which can cost USD $15k-$75k
And Internal Project.
In this mode, your app is limited to G Suite users within your organization . If you select sensitive or restricted scopes, your test users will see an 'unverified app' screen before they use your app. You can communicate with your internal users directly about how you'll use their data.
It does not allow me to select internal, only external project.