There is a convention already, and it's quite simple. There's an "autocomplete" property on an input tag, and you can tell the browser what it's for[1]. Use "username", "email", and "current-password" for example when building a login page. ("new-password" is useful for creating an account or changing password, and the password manager can suggest a new password here instead of trying to autofill with your old password).
The autocomplete attribute supports nearly everything you can imagine. Check this for a full list[2].
[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/In...
[2] https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes...