I hate to be so negative but, having spent literally weeks trying to get a Xamarin application to deploy to a physical Apple Watch, I believe that this is not currently possible. To speak even more frankly, I think this 'feature' should either be removed from Xamarin or properly supported.
For over a year, I have been working around various Xamarin foibles on a new tech startup project that is literally the difference between my having a future and not. So, when I chose to put my faith in Xamarin, the stakes could not have been higher.
Initially, I paid the full whack for a Xamarin licence. Both before and after Microsoft bought Xamarin, I have generally been satisfied with the support I have received, both here on this forum and from Xamarin. But the Apple Watch support is a different story. To be fair, I have received help from various people with this, but the fact remains that here I am, six months after first looking into the Apple Watch on Xamarin, with my whole project and future dead in the water.
I have followed every single guide and troubleshooter out there. I just cannot get a Xamarin application to go on to a physical Apple Watch. I managed it just once. I even hit a breakpoint in the debugger on the PC! I thought, "Great! Now I can begin work tomorrow morning." The following morning, without touching a line of code or a single setting, the whole thing was broken again. And I have never got it going since that day, which was about two months ago now.
Generally, I remain impressed by Xamarin, but , please, please, please can someone take a look at the deployment of code on to a physical Apple Watch? It is unusable as it stands. And therefore, as it stands, my new business doesn't have a chance and I am forced to admit that my time might have been better spent simply learning to use XCode on the Mac.
Yours desperately - A Loyal Xamarin Developer