this works

curl -u admin:smartvm -O -k -v -X POST https://192.168.1.17/api/services/99000000000001/custom_attributes \
-H "Resource:" \
-H 'Content-Type: text/json; charset=utf8' \
-d @- << EOF
{
        "action": "add",
        "resources": [{
                "name": "ip-10-0-1-217.ec2.internal",
                "value": "10.0.1.217"
        }, {
                "name": "ip-10-0-1-192.ec2.internal",
                "value": "10.0.1.192"
        }, {
                "name": "ip-10-0-1-98.ec2.internal",
                "value": "10.0.1.98"
        }]
}
EOF

But this

- name: Find the generic objects that belong to this service
  uri:
    url: "https://localhost/api/services/99000000000145/custom_attributes"
    method: POST
    validate_certs: no
    body_format: json
    body: "{{ lookup('template','templates/custom_attributes.j2') | to_json }}"
    validate_certs: false
    user: admin
    password: r3dhat2018
  register: cra

{
	"action": "add",
	"resources": [{
		"name": "ip-10-0-1-217.ec2.internal",
		"value": "10.0.1.217"
	}, {
		"name": "ip-10-0-1-192.ec2.internal",
		"value": "10.0.1.192"
	}, {
		"name": "ip-10-0-1-98.ec2.internal",
		"value": "10.0.1.98"
	}]
}