this is most certainly a bug. I managed to work around it though.
Go to your firewall api page "https://<FW MGMT IP ADDRESS>/restapi-doc/#tag/key-generation/paths/~1api~1?type=keygen/get
Use postman (or any other api tool) to generate the API key for ADMIN user. and copy the API key that's returned.
switch to the expedition and when trying to add "Authentication API Keys" - switch the "Auth. Type:" from "username and password" to "API KEY"
Paste the API key here.