To get the coordinates of the upper right corner of your map, use the following expression:
item_variables('Karte') points at the item you want to work with. You need to replace
Karte with the name you gave your map item.
map_get([...],'map_extent') grabs the extent of the item you're pointing at. In this case you're returned a geometry.
Once you get the geometry, the rest is rather easy:
x_max followed by the geometry you need the coordinates of to grab said coordinates. Combine min/max and x/y in order to create the fitting coordinate pairs.
||', '|| concatenates these coordinates.
I added the
round function since I tried this using a UTM-zone and didn't want unnecessarily long numbers. If you're working with DMS-coordinates, please remove
round since it'd probably break the expression.