[cyphar]

Password managers that have browser integration already function this way -- you have to go out of your way to copy-paste your password. The main problem is that some sites design their login forms to make this kind of functionality harder (such as putting the password and username fields on different pages, or having strange layouts where you need to also input your last name, and so on).

I personally use KeepassXC which has a browser plugin that does this for you (and it's nice that the plugin doesn't have access to your passwords directly -- it has to request access from the password manager which be default gives you a popup asking for permission to share specific credentials).