From New Outlook (client) launch URL in system browser

From an Outlook Addin running in New Outlook (client) we need to be able to open a third party website in the system browser. This is currently only possible on New Outlook on web. Only method currently available is to open a dialog, but the third party site does not fit very well there.

I am rewriting an old VSTO addin to be able to run in New Outlook. The addin opens a third party web site that I do not control. I need to be able to replicate the behavior of the VSTO addin which calls ShellExecute WIN32 API to handle the URL.