How to use a Custom Hosted Login Page for Universal Login

There are a couple apps that I am trying to use a Universal Login Page for.

Following these docs. Implicit Flow with Form Post

I have previously set it up before enabling a Custom Login Page and doing authentication through there.

But in this new case. We have created and extracted and extended the code to our own domain with custom branding and extra functionality that was needed.

At the moment I can not figure out if there is away we could call webAuth.authorize() and have it redirect to our {MyCustomHostedDomain}/login page and authenticate there. If I try to update the login page to forward to our {MyCustomHostedDomain} and try logging in I then get a crossdomain post issue.

The only other way I know to make this work is to redirect users manually to our {MyCustomHostedDomain}/login page then use this Authentication API Explorer to have them login/grab a token and return to the call back.

So my question is whats the proper way of getting this flow here

working on my {MyCustomHostedDomain}/login. Or is the only way to just paste it into a single file in the universal login template.