To create a secondary contact, the user should have permission to manage deals.

{
	"contactDeal": {
		"contact": 1,
		"deal": 2
	}
}

{
    "deals": [
        {
            "hash": "c7b5a271",
            "owner": "1",
            "contact": "2",
            "organization": null,
            "group": "1",
            "stage": "1",
            "title": "deal1",
            "description": "",
            "percent": "0",
            "cdate": "2018-10-31T13:33:09-05:00",
            "mdate": "2018-10-31T13:33:09-05:00",
            "nexttaskid": null,
            "value": "10000",
            "currency": "usd",
            "winProbability": null,
            "winProbabilityMdate": "2018-10-31T13:33:09-05:00",
            "status": "0",
            "activitycount": "1",
            "nextdealid": "2",
            "edate": null,
            "links": {
                "dealActivities": "/api/3/deals/2/dealActivities",
                "contact": "/api/3/deals/2/contact",
                "contactDeals": "/api/3/deals/2/contactDeals",
                "group": "/api/3/deals/2/group",
                "nextTask": "/api/3/deals/2/nextTask",
                "notes": "/api/3/deals/2/notes",
                "organization": "/api/3/deals/2/organization",
                "owner": "/api/3/deals/2/owner",
                "scoreValues": "/api/3/deals/2/scoreValues",
                "stage": "/api/3/deals/2/stage",
                "tasks": "/api/3/deals/2/tasks",
                "dealCustomFieldData": "/api/3/deals/2/dealCustomFieldData"
            },
            "id": "2",
            "isDisabled": false
        }
    ],
    "contactDeal": {
        "contact": 3,
        "deal": 2,
        "cdate": "2019-02-25T09:00:02-06:00",
        "links": {
            "deal": "/api/3/contactDeals/3/deal",
            "contact": "/api/3/contactDeals/3/contact"
        },
        "id": "3"
    }
}