To get the coordinates of the upper right corner of your map, use the following expression:
round(x_max(map_get(item_variables('Karte'),'map_extent')))||', '||round(y_max(map_get(item_variables('Karte'),'map_extent')))
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:
Use y_min, y_max, x_min and 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.