How do I add a value to an existing custom property when creating a node?

Hello. I'm using the Python SDK to create nodes and would like to add a value to an existing custom property. The property is "Department". I tried adding that to the properties dictionary that gets passed into the swis.create method as an argument. I'd get an invalid column name error when the method is invoked.

My question is what is the correct way to add a value for an existing custom property? Do I create a separate custom properties dictionary? Which schema do I use? Thanks.