That popup is generated when you include more than one MediaObject in the MediaObjectsForURL() function of the URL Service. Most Plex client apps (generally the mobile ones) skip the popup and make an "intelligent" selection based on user settings, wifi vs 3/4G, local vs remote, etc. PHT (and I believe Plex/Web) have settings to auto-select from the list and skip the popup as well.
If you're not using an URL Service, it will be a little more complicated but should still be do-able. The main idea is to return a list of MediaObjects each with a different resolution from the method that is constructing the VideoClipObject. I can't think of any examples where this is done without an URL Service but if you look in the Services.bundle, there are lots of examples that use it there.