2FA with ODP on Android Phone

I might be doing an obvious mistake: I just tried to add my android-phone as second authenticator for login into my self-hosted gitea. I am using the app andOTP for this.

In the settings off my gitea-profile, I click “Add Security key”. A QR-code shows. andOPT fails to read it, but I can use a qr-code-reader, and I copy the resulting link into andOTP. The entry now shows a 6-digit code.

Now the issue is, that gitea doesn’t show a inputfield in which I could enter this code. Usually, I’d be confirming the new key by entering the 6-digit code into the webinterface of gitea. But it only shows the qr-code.
What am I doing wrong?

Adding a hardware USB-key was not a problem.

Thanks for helping.