Hi Kevin,

The URL set on the form created by BeginPaymentForm comes from the payment provider, but from what you've said, I'm not sure this is the one you are having problems with.

It sounds like it's the return, cancel and callback URLs which are indeed worked out from the current request.

As of the latest release, our code for working out the baseUrl is

var xForwardedProto = true;
var xOriginalHost = true;

string scheme = request.Headers["X-Forwarded-Proto"];
if (scheme.IsNullOrWhiteSpace())
{
    scheme = request.GetScheme();
    xForwardedProto = false;
}

string host = request.Headers["X-Original-Host"];
if (host.IsNullOrWhiteSpace())
{
    host = request.GetHost();
    xOriginalHost = false;
}

int port = int.Parse("0" + request.Headers["X-Forwarded-Port"]);
if (port == 0)
{
    string cfVisitor = request.Headers["CF-Visitor"];
    if (!cfVisitor.IsNullOrWhiteSpace())
    {
        port = cfVisitor.InvariantContains("https") ? 443 : 80;
    }
    else
    {
        port = xOriginalHost
            ? xForwardedProto
                ? scheme.InvariantEquals("https") ? 443 : 80
                : 80
            : request.GetPort();
    }
}

GetHost and GetPort or just wrapper extensions that wrap differences between .NET Framework / .NET Core.

This does of course depend on what version of Vendr you are using as the CF-Visitor stuff was only added in a recent update.

What do you have in front of your server?