HTTP vs HTTPS

Hi everyone !!

I have a web application deloped on a web server. I have site A and site B both have link to this application. I want to check the URL from A and B. If it's correct with my allowed URL then I will let them advance. There is a sittuation occurs. Application A use HTTP and application B use HTTPS. I use Uri RemoteURL = Request.UrlReferrer to check if the URL is really from site A and site B and then match with my allowed string. Site A can pass the check point but B.

I was wondering since B use HTTPS therefore it could not go through

I may ask unclear. Help me if you can

Thanks