EMB-505 First Officer reported the Cabin Pressure increased uncontrollably in flight. The flight crew was granted a descent from ATC just as the Cabin Altitude High CAS message was displayed; and the flight landed safely at the destination airport.

Narrative

During climb-out Person A and I noticed a rise in Cabin Pressure. During this time we continuously monitored the Cabin Rate and Cabin Pressure. Between switching the Cabin Pressure between manual and automatic we were able to briefly stabilize the pressure from increasing too rapidly. As we continued our climb the pressure became more uncontrollable through the use of the manual mode and it eventually began to rise rapidly towards a Cabin Altitude High CAS message. During all of the events above; Person A and I were using great communication and decision-making on what to do if we started to get closer to that Cabin Altitude High CAS message. Communication with ATC about a descent request was established well before the CAS message displayed. Simultaneously we began our descent and the CAS message went off.Both pilots noted and continuously monitored the situation. Good communication between the two of us on what to do and when to do it. Cabin Altitude High CAS message displayed. The cause of this event was due to a Lavatory Cap that had become dislodged from its original position; causing a pressure leak from the valve located in the toilet. While in talks with ATC we were given a 'standby; traffic' on our original descent request before the Cabin Altitude High CAS message displayed. After another attempt to get a descent from ATC we were granted to descend and at that time we received a Cabin Altitude High CAS message and continued our descent and both pilots agreed to descend into ZZZ; non-emergency. If this event were to happen again I would have requested a descent earlier.