How can I tell a password manager which username/password to autofill with?

Question

Question

How can a webform tell the browser which username and password are appropriate inputs when autofilling if there are multiple entries for the same (primary) domain?

Context

I design an authentication system that offers multiple identity providers, and most of our users have accounts on more than one identity provider (IDP). These IDPs are on different subdomains (but I'd rather not limit answers to that, if possible). A username is only unique within an IDP, not across all IDPs.

This has led to some usability problems

1. The password manager may autofill a username and password that isn't appropriate to the particular identity provider someone is logging in with. For example, browser fills in redacted_username1 when logging in to subdomain4.example.com.
2. The password manager may suggest only one username and password out of many, and the wrong one at that, requiring the user to search through the password store for the matching one.

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