Thanks for the tip!

I had to make one small change because my callback path was "/signin-microsoft"

On the RegEx I modified it to the following:

Regex.Replace(context.RedirectUri, "redirect_uri=(.)+%2Fsignin-", "redirect_uri=https%3A%2F%2Fwww.yourcustomdomain.com%2Fsignin-")

Note the "signin-" instead of "signin-oidc"