Custom URL field throwing error from API

I'm working on a syncing script between our organizations GitHub and Jira accounts, and issue creation is happening until I include the custom field that references the GitHub issue.

{ 
 "labels": [], 
 "project": { "id": "10000" }, 
 "priority": { "name": "Medium" }, 
 "description": { "type": "doc", "content": [ { "type": "paragraph", "content": [ { "type": "text", "text": "..." } ] } ], "version": 1 }, 
 "issuetype": { "id": "10002" }, 
 "components": [], 
 "summary": "Github Title", 
 "assignee": { "name": "Github/Jira user association" }, 
 "customfield_10021": { "value": "https://github.com/org/repo/issues/1" }
} 

The custom field I have linked to these issues is confirmed and works within the interface, but when I submit via the v3 rest API I get the following error response:

{ 
    errorMessages: [],
    errors: { customfield_10021: 'Not a valid URL' } 
}

 I've tried all variations of the custom field I could think of (mainly the above, and passing a regular string, which tells me it needs to be an object).

These API errors and documentation really seems lacking in clarity...