Is there a reason you want to use the API?  Or that you wouldn’t consider a Workflow rule?  I used the API for reporting purposes, and for system data updates (from our financial system).  Anything internal to AdaptiveWork was easier to manage via workflow rules I found?

If you need to use the API there’s good documentation for it here: https://api.clarizentb.com/V2.0/services/  You start with the login (user/pass), then you get the Session ID which you can pass in subsequent POST requests in the header.

I have gone through the documentation but not able to identify any such API.

Is there anywhere else I can search for the relation API?

You might need to provide some more detail into what you are actually trying to do in the use case sorry??

I have a project created through API and I have also created one issue. I want to link them through API. So, in the related items tab on UI it is available.

From the https://api.clarizentb.com/v2.0/services/metadata/describeEntityRelations endpoint, I am able to find that the relation name is “RelatedIssues”. But how to add it through API is not mentioned in the document.

I have also tried to add the relation through UI and its available by this https://api.clarizentb.com/V2.0/services/data/relationQuery endpoint.

Is there any way in API I can add the relation through reference field and send it through update request?

If you have the reference to Project object you have created, and the Issue Object that you have created then you would need to also create the “RelatedIssues” Object in the same way that you created the Project/Issue - were you using CreateandRetrieve? 

In the AdaptiveWork Ui Configuration it’s in “All Links” then “Related Work”.

I think you would need to pass the Work Item ID (project), and the Case ID (Issue) both of which you should have reference to from creating them. And you may have to set the Item Type, which would be “Related Work” I believe (though this might default without having to set it?).