From my experience, i had to initially choose the pulse app to be the default message app at the very first in the beginning. It then has permission to set up syncing and all of that. Immediately after, I changed it back to Message+ or anything else, but the pulse app would still send the new texts to my smartwatch, even if it wasn't the default messaging app on my phone. I think that's all it's calling for. Just the initial access making it the default gives it, then you can change it back but the pulse app would still have configured things to continue working -- it most likely has something to do with the way it "syncs" the messages across "all devices (including computers)" and just needs to set certain things up intially, then you could supposedly go on using your own preferred afterwards...

hope this helps...