Can I pass in Auth credentials with a URL when setting up alerting from a synthetic alerting provider?

Yes - Insert your username (umpire) and your password (found on the alerting tab of the section.io portal) at the front of the Umpire Check URL

like so

https://umpire:password@aperture.section.io/account/xxxx/application/xxxx/umpire/check?metric=metrictobechecked

We have checked a number of the major synthetics providers to confirm this works. It is possible that not all providers support this feature.