Ok, it looks like I was wrong, and Julian identified the correct code here: https://forum.xojo.com/conversation/post/388637
Here's the newer version of the code which appears to work
// this is an updated version of the function which handles both 64 and 32 bit builds
// for Xojo 2018 R1 or later
Dim reg As New RegistryItem("HKEY_CURRENT_USER\SOFTWARE\Microsoft")
reg = reg.AddFolder("Internet Explorer")
reg = reg.AddFolder("Main")
reg = reg.AddFolder("FeatureControl")
reg = reg.AddFolder("FEATURE_BROWSER_EMULATION")
' documentation : https://docs.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/general-info/ee330730(v=vs.85)
reg.Value(App.ExecutableFile.Name) = CType(11000 , Int32)
Testing this is tricky, because if you run the 64 bit app immediately after the 32 bit app, and the registry key set by the 32 app is still there, the 64 bit app will appear to work. Use RegEdit to clean out the registry keys between tests.
@Paul L - we'll need the documentation updated again as soon as we confirm that Julian's code is correct.