The window name is Garmin Mobile PC. Instead of the details on the button use the External GPS in RRconfig like this:
Also see this thread
Then you can use the "GPS" command to open external_GPS.skin. Again I suggest using the Carwings skin as a test.
Now if you could only explain how I get my osk skin to actually send commands to garmin that would be great. I looked at the options in the editor but its not really making sense.
For a OSK skin, you should be sure to use the "apposk" command to load it, but there's no guarantee it will work (just depends on the application itself)...
You can also try sendkey like below:
"ACTIVATE;Garmin Mobile PC||SENDKEY;a"