UTM coordinates are not enabled yet, but they will.
As for your question:
Easy way: You input the 10 digit grid coordinates (5 for Eastings and 5 for Northings).
Hard way: When the location is not in the current grid but in an adjacent one, then you have to input 11 digits. First digit tells to which UTM segment the coordinates belong.
The INS always create a grid of UTM segments around the aircraft. You do not need to know the UTM prefix, you only need to select the segment by entering its number. For reference use the UFC keypad. 5 is the current UTM segment where the aircraft is located. The numbers around the 5 are the adjacent UTM segments (1,2, 3, 4, 6, 7, 8, 9). Each UTM segment is a 100 x 100 km square grid.