Account credentials will usually be automatically added to your Saved Passwords (i.e., your Keychain) when signing-in to a service for the first time from your iPad. You will usually see a prompt to save the credentials.
When manually adding new credentials to your Saved Passwords, the Add Password page will automatically create a complex password for the entry. If you wish to use a different Password (presumably because the account already exists elsewhere), simply tap and edit the Password field before saving with the Done button.
This tutorial, from MacObserver, may help with general management of your Saved Passwords:
https://www.macobserver.com/tips/quick-tip/manage-keychain-passwords-iphone-ipad/