Here's an example
I have a custom item type called CM5_MyItem with CM5_MyItemRevision which has custom properties cm5_Property1 and cm5_Property2
In Teamcenter BMIDE the create descriptor has these properties, which means I can assign them during creation.
To do this in Mendix, you need to create an entity like the create descriptor i.e.
This is then used as input for the "Create Object" activity when creating your new item revision.