When you click on the waypoint description (version 4.1) and you have the Pro version, an edit dialog will be shown. The possible coordinate formula is marked in darkgreen and underlined. You can correct this by selecting text and pick 'Mark' from the menu. The resolved formula is shown in the edit field.
In the example, the value for 'A' is already given (in this case '4'). Since not all characters are available yet as value, the coordinate is not solved yet. As soon as it is solved (thus you specified the correct value for 'B'), you can lock the coordinate (click the lock icon) and the waypoint is shown on the map.
The solved coordinate can be modified. Be sure to click the lock afterwards to pin it, otherwise it will be reset after a new calculation.
A guess is made which character needs to be given. Be free to pick another.
Note that formulas expect a coordinate, so you might have side effects if a final location contains E/W/N/S characters.