Currently, Textra SMS does not have it's own stand-alone Desktop or Browser based application.
However Textra is integrated with both PushBullet
, which you can install from 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!