Which email clients support sending the input password as: pwd+top code?
It's a pretty commonly used, and works very well, but requires user education on how to fill in their combined password. A proper API with distinct fields for password and OTP is cleaner, but requires protocol support.
A dedicated question for the OTP would be much better. Also, the password manager would know to not save the password+OTP every time as a new password.
It is, but think of why you'd build this. You own the backend and need to add 2FA support. The various client software isn't written by you so you can't change them. This approach allows the client software to add an OTP field (concat the fields for the user) but doesn't require it (user must concat OTP on password manually).
Many of the places I've seen this used don't integrate well with software password managers. OS login screen, console apps, etc; typically not web apps. But this is a good criticism.
instead of sending your password as johndoe, you send johndoe123456 where 123456 is the code