Associate a custom object record with Contact Object Record

Summary:

Associate a custom object record with Contact Object Record

Content (required):

Hi Guys,

I am working on integrating Service Cloud with an external system. The external system will either create or update a custom object which is a child of Contact. I have created a Custom object and relevant workspaces. I am also able to create a custom object record (manually through the workspace) and associate it with a contact(also manually doing this on the workspace). I have created a custom report and added it as a tab in the contact workspace. When i create a record through postman, using the custom object api (/services/rest/connect/latest/Custom Object), I am unable to do an automatic