DllImport:RegisterCallback(pbc, url) and BindMonikerToStream(pmk, pbc)

I'm trying to use this post so I can get the URL of file downloads that are not exposed with the HTML or the webbrowser.navigate event. The code is incomplete in that example though. The answer makes reference to

RegisterCallback(pbc, url)
BindMonikerToStream(pmk, pbc)

which are not defined. I can't find proper definitions for those methods online. There are some, but with different arguments. pinvoke.net doesn't list RegisterCallback or BindMonikerToStream, but DOES have CreateBindCtx.

Does anyone know how those two should be defined?