After talking to the adventuring party in Trademond during Reverse Summoning Ritual, go find Desmond in Verdeaux in front of the tower south of the market square.
First, enter the tower either by paying a 250 G fee or by being on Roland's lewd path.
Once inside the den, talk to the Entertainment mage. You can complete the quest either by paying 5000 G or by taking his challenge. If you take the challenge, you don't need to win to obtain the contract (but losing leads to a specified penalty). The four battles during the challenge are:
You don't need to win to get the contract. However, if during the battle you get over >=100 applause you get the contract for free. You start at 0.