I was wondering the same and I see that it is an old thread but I'll still reply if anyone needs help regarding this.

I was setting up Google Calendar API for myself as well (Obsidian integrations and stuff) and wondered the same thing. If you'd like to add & use Sensitive Scopes in your 'project' without Google's approval (or setting up websites etc.). Simply:

1. Head to Google Auth Platform -> Audience -> Set it to Testing.
2. Under 'Test Users' add your own Google email.

Also, after setting this up, I believe you can use https://developers.google.com/oauthplayground/ as well but tokens need to be refreshed (can be easily set-up by using your own Oauth credentials (⚙️ Gear on the top-right corner).

It worked like this for me with Google Calendar's sensitive scopes.