Für mich hat folgendes Funktioniert:

{
    "jsonrpc": "2.0",
    "method": "cmdb.category.create",
    "params": {
        "objID": "1337",
        "category": "C__CATG__CONTACT",
        "data": {
			"contact": 42, # PersonenID
			"role": 2, # Administration > CMDB Settings > Roles administration
			"primary": 1 # Primärkontakt
		},
        "apikey":""
    },
    "id": 1
}

Das Ergebis:

{
    "id": 1,
    "jsonrpc": "2.0",
    "result": {
        "id": 4242,
        "message": "Category entry successfully created. [This method is deprecated and will be removed in a feature release. Use 'cmdb.category.save' instead.]",
        "success": true
    }
}

Leider funktioniert bei mir nur "cmdb.category.create" statt "cmdb.category.save"