Willing Slave Molly is one of the vast amount of routes in Adventure High.
To access it, the Protagonist has to sign the contract without having enslaved neither Jane nor Molly prior. After signing it, choose to fulfill it and enslave Jane instead by casting Suppression on the line "I want you to cast Suppression-" to enslave Jane.
After doing so, head to the lake and speak with Molly. Push for more details and then tell her that you do you want her.