Hi ​@Hoveys,

When checking an existing ticket with a parent/child relationship, you'll see the parent ticket ID listed in the properties. To attach an existing ticket as a child to another, you simply need to replicate this structure in a PUT call.

Alternatively, you can only create a child ticket directly from the parent ticket using this API: https://api.freshservice.com/#create_child_ticket

Best regards
Daniel

something like this ? 

{
    "related_tickets": {
        "child_ids": d
            748
        ]
    }
}

Then I get this: 

{
    "description": "Validation failed",
    "errors": r
        {
            "field": "related_tickets",
            "message": "Unexpected/invalid field in request",
            "code": "invalid_field"
        }
    ]
}

​@Daniel Söderlund  I think you're right — I might be wrong. I just created new tickets via the API (with attaching to an existing ticket), but it looks like the API doesn’t support manually attaching an existing ticket. Looks like a missing functionality. 

Maybe you have another idea.

Project has it’s own endpoint for relating tickets/change etc to a project task and it works great. 
Something like that with the rest of the modules would be great. 

​@Daniel Söderlund & ​@DanielRuff 

That's correct; the Freshservice API only allows you to create new child tickets and does not provide a feature that would allow you to link two existing tickets in a parent/child relationship. I have confirmed this with Support, who have informed me that there is currently no option to link existing tickets.