As of now, there is not a FollowMyHealth™ app for Windows phones or other Windows devices such as Surface Pros. You may receive a message that states "This application is not supported on mobile devices. Please log in with a desktop computer." However, you can still access your FollowMyHealth account. The trick is that you have to use the desktop version of FollowMyHealth on your Windows device.
What you can do is go to www.followmyhealth.com first. Then, go to your settings in the browser you are using, whether its IE or Edge or another browser. You should see an option for website preferences. Once you find that option, switch it to Desktop version. Then, go back to FollowMyHealth and refresh the page. This will then allow you to display FollowMyHealth just like on a computer or laptop.
Note that it may be a little harder to navigate since it is meant to be viewed on a desktop/laptop. If you are using a larger Windows device, such as a Surface Pro, you may be able to have the full desktop browsing experience.