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function createNewOpportunityFromCase() {
    var xhttp = new XMLHttpRequest();
    xhttp.open("POST", "services/apexrest/CaseToProject", true);
    xhttp.setRequestHeader("Authorization", "Bearer {!$Api.Session_ID}");   
    xhttp.setRequestHeader("Content-Type", "application/json");

    var payload = {"caseId" : "{!Case.Id}"};

    xhttp.send(payload);
} 

createNewOpportunityFromCase();