How to test widget on device?

Here's a newbie question for you:

I have a widget that can be tested on a VivoActive3 by just copying it to /GARMIN/APPS. But doing the same on my Forerunner 955 does not work.

I expected the widget to show up next to the default widgets, but it is nowhere to be found. Furthermore, the .prg file is removed.

How can I test my widget on the FR955? (I'm using SDK 4.1.5).