For apps like that, not only will it not offer a suggestion, it probably won't remember whatever you put in it manually.
For things like this, I either try to use their web-page equivalent, which at least will get it into Keychain so I can look it up manually next time, or if there is no web option, you can make keychain generate passwords manually, though you still have to do this manual search for it each time, then copy/paste to the app…
From Keychain Access, + button or Cmd ⌘ N
Give it a memorable name, account name & then click the key button lower right.
You can choose length & password type.
Once completed click Add.
Alternatively, if you also have an iOS device with the same app & password if you fill it in there manually, then Keychain will remember it, but only auto-fill it for iOS, not for the Mac app.
File this under irritating. ;)
Late thought - watch out if you get iOS to add it to keychain after doing this manually from macOS, because you will likely end up with two independent entries.