Currently, Textra SMS does not have it's own stand-alone Desktop or Browser based application. 

However, Textra SMS is integrated with both PushBullet and MightyText, which you can install from the Google Play then follow the instructions for how to also set them up as a Desktop application or as a plug-in to your web Browser.

In addition for Windows 10 Desktop, you can use the Microsoft Your Phone app to control your Android apps directly as a window on your Desktop. Just download and start using it!