CustomPhysicalProperties expects a PhysicalProperties datatype as its value. I do not believe the properties of PhysicalProperties are directly mutable so you need to construct your own PhysicalProperties and set it to CustomPhysicalProperties if you want to override values like friction.
PhysicalProperties is a creatable datatype. CustomPhysicalProperties is the property of BaseParts that you write to. Please refer to CustomPhysicalProperties for a code sample, it’s at the bottom of the page.