The chest is filled up with reward money from the quests completed by heroes. Essentially, you pay heroes for completing the quests, then you get payed for having gotten the quests taken care of by the clients. To fill the chest up all you need is to assign quests with higher rewards on them to heroes.
Note that you need to pay out the hero yourself until the client reimburses you for the quest, so the higher reward a quest is, the more you have to pay out to the hero. The pay split between hero / guild is meant to be based on quest preferences of the hero, but is unimplemented so it is a random value from 25% - 50% of the quests total reward.