Here's the upper-right of my Garmin Connect screen (grey circle to hide my profile picture):
There is ample space (yellow rectangle) to add a "Manual Activity" button here. This option is currently only available by clicking "Activities" and then "+ Manual Activity."
There is no reason to make adding manual activities a second-class citizen in the interface. Why does uploading an activity get a handy button and not adding a manual activity? The Strava interface treats both options on par. This seems right to me.
The same problem also shows up in Garmin Connect on mobile.