var oCustomProperty = Selection(0).AddCustomProperty("whatever");

// add parameters
oCustomProperty.AddParameter( "number", siFloat, siClassifUnknown, siPersistable, "", "", "", 0.5, 0, 1, 0, 1 );

// hide Name
p = oCustomProperty.Parameters("Name")
p.SetCapabilityFlag( siNotInspectable, true );