Using an external URL for Multi-factor auth

[Brian Mathis]

I have a system that uses a simple GET request to validate a
multifactor token.  Validation success is signaled by returning an
HTTP status code.  How can I use this with Shibboleth IdP (3.2.1)?

I've looked at the ExternalAuthnConfiguration and I am not sure if
this is what I need to use?  Is the shibboleth idea of "external" the
same thing as what I'm talking about here?  The user's browser
shouldn't be forwarded to the external system; it would be shibboleth
acting as the client to the validation service on the backend.

If not, is there any boilerplate code that does something like this?
I'm mainly a sysadmin, but I can understand code at a basic level.

Thank you.