If you know the coordinates you want to use, you’ll have to input them as part of a full pasted flight plan. You cannot search for them, and they won’t appear as waypoints on the map (because they’re not waypoints).
The only exceptions are those waypoints which fall on round longitudinal and latitudinal numbers, like 60 N, 20 W over the Atlantic:
These take some playing around with, but they’re on the map, so you can search for them.
For example, 60N20W would be found using 6020N in the search bar, while 60N20 will give you 60N, 120W. 60W20 gives 60 S, 120 W.
While these waypoints can be found, they’re really only useful for transoceanic flights which use them as a matter of course. For general flight plan creation, the system in the table up top is the way to go. That’s the only way to get specific GPS coordinates that don’t happen to fall on a whole number both longitudinally and latitudinally.