Not all login pages will be detected as such by your iPad (some are seemingly deliberately contrived to avoid detection by Password management features) - and as such, the opportunity to automatically create and save an account username/password may not appear. Similarly, some websites also don't work with Autofill.
Neither of these situations prevent you from manually saving account credentials to your Keychain. Assuming that your iPad is running iPadOS 17 or earlier, account credentials can be easily added to your saved passwords in iPad settings:
Settings > Passwords - tap the blue "+" icon at top-right of the settings page, then select New Password
Enter the website URL, your login Username (this might be an email address) - and either accept the automatically generated Password, or erase and enter your chosen Password. Add any relevant Notes that you wish to store with the entry - and tap Done when you're finished.
If your iPad is running iPadOS 18, saved Passwords are still saved in your Keychain, but are no longer accessed from iPad settings. For iPadOS 18, these a managed from within the new Passwords App:
Use the Passwords app to create, manage, and share passwords and passkeys across Apple devices - Apple Support