curl -n -X PATCH  -H "Content-Type: application/json" \
    https://example.com/confluence/rest/projectdoc/1/document?id-list=12845218,12845219,12845221 \
    -d @insertProperties.json

Using curl -n assumes you are using a .netrc file to authenticate. Please see REST Login to Confluence with cURL for more options.

 {
    "property": [
      {
        "name": "Customer Support URL",
        "value": "<a href=\"https://example.atlassian.net/servicedesk/customer/portal/1\">Customer Support</a>",
        "controls": "",
        "position": "after",
        "ref": "Short Name"
      },
      {
        "name": "Service Contact",
        "value": "<a href=\"mailto:support@example.com\">support@example.com</a>",
        "controls": "",
        "position": "after",
        "ref": "Iteration"
      }
    ]
}