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Hi @Hundare, N.V. (Nikhil) Greetings! Welcome to Microsoft Q&A forum. Thank you for posting this question here.As per the documentation of the API Pull Requests - Create the repository ID in the API URL has to be the repository ID of the pull request's target branch.
In your case the URL request has to point to the upstream repo Id but not the fork repo Id. The repository property of the request body holds the information on the target branch of the pull request, i.e., upstream repository. Since you are passing the Fork repo Id in the URL and passing the upstream branch Is in the repository property of the body, the request fails due to mismatched Id.
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