Updating Domain on Org Change

Hello Team, 

I am looking to solve an issue of having multiple domains for my customers, yet only a single domain in our SFDC instance.  Every time it is updated it overwrites any additional domains we've added.

I was thinking of creating a new org field to host those extra domains names, and then a trigger that when an org is updated we hit the ZD webhook to update the org domain_names field to include the SFDC one and the domains in additional domains.

However I am trying to test my webhook against /api/v2/organizations{{organization_id}}, with the following and get a 422 unprocessable error,  this works with plain text.

{
   "organization": {
       "domain_names": "{{organization.custom_fields.xxxxxxxxx}}"
   }
}

I assume this is because there is something wrong with my placeholder, can we call directly from the organization if don't have a preceeding user or ticket?