The default rule for answer files is leaving any required field blank, will force Windows to stop and ask you for missing details.
But that doesn't work for the product key. From my testing:
1. Leave the product key blank. NTLite defaults to this, and Windows doesn't stop to ask.
Listing <InstallFrom> overrides the need to ask for a key. Setup expects you to install the same edition that matches your key, so specifying which image overrides the need to check your key for that info. This doesn't work.
2. Leave <InstallFrom> blank, by using "Prompt edition selection". This doesn't work.
3. Disable networking, to prevent Windows from self-activating off a previous digital license. This doesn't work.
4. Not having an answer file does stop for the missing key. But we're not using unattended mode.
I
n short, there's no way to force W10/11 unattended to prompt for a key. This might have stopped working in 20H2.