Remove the seeds from the melon pulp. Cut out small balls with a ball cutter. Put liqueur and melon balls in a large bowl and marinate for about 1 hour.
Wash 1 lime hot, grate dry and cut into thin slices. Squeeze 2 limes. Add lime juice and sugar syrup to the melon balls.
Wash the lemon balm, shake dry and cut the leaves into strips. Before serving, add lemon balm and lime slices and top up with prosecco.