I find it the easiest to first save the login in BW and then paste to the signup form. It is the fastest method I could come up with. The worst thing that happened (actually more than once) was, that the password was too long. But a new password is generated very quickly.
It is also the safest way because, by doing it this way, if the computer crashes you already have the credentials stored in Bitwarden and can setup the login when the computer is working again.
If people do it the other way round then there is the possibility of setting up a login which doesn’t get saved in the password manager, because the computer crashed before it could be saved.
This may seem strange to those who have used inferior password managers, but it is logical.