Talking out my butt, but don't think can force, as believe Safari is keying off the field type on the webpage. Ie, if type and or name is not something like "password", no joy. See something similar but in reverse on my bank: login id is called something like Member ID and won't autofill from Keychain, but password field works ok from Keychain.
The hacks, on Mac, bring up Keychain and do File > New Item (Entry?). Go to password field and generate one and copy/paste. Like this better in that can make how long I want, special chars or not, etc.
But since I use a password manager, I use their built-in generator and copy/paste to account create screen. Two birds with one stone (update pw manager, create strong pw).