Adding a keychain entry

How do you add a web login address, user name, existing password to keychain?

Would like to do this on a M1 equipped iMac running Monterey 12.1 using Safari Version 15.2 because I currently use another password manager and currently have a problem with it only on my 2021 iMac but not on my 2019 MacBook Air. When using copy and paste I am able to login to every web site with my credentials but never have the option to save this Information in keychain.

Have explored using Apple Utilities>Keychain Access & Non-Apple vendors Keyholder Access (free in Apple App Store) but stymied on both. Have seen a demonstration doing this in Keyholder Access in a local users group, but I am currently unable to do this because of legacy home products.