update lookup field with value from text field

I would like to update a caseNumberId, which is a lookup field with ticketnumber from a text field when case is saved.

I have the following function on Case onLoad event:

function saveCaseHiddenFields()
{
     var formType = Xrm.Page.ui.getFormType();
    if (formType != 1)  //on update
    { 
          var caseId = Xrm.Page.data.entity.getId(); //case GUID
         
         if (caseId)
         {
              var caseNumberId = Xrm.Page.getAttribute("ticketnumber").getValue(); //get ticketnumber
       
              var caseNumberLookUp = new Array();
              caseNumberLookUp[0] = new Object();
              caseNumberLookUp[0].id = caseId;
              caseNumberLookUp[0].name = caseNumberId;
              caseNumberLookUp[0].entityType = "incident";
 

             var updateCase = {};
            updateCase.dads_caseNumberId = Xrm.Page.getAttribute("dads_casenumberid").setValuecaseNumberLookUp);
            SDK.REST.updateRecord(
                     caseId,
                     updateCase,
                     "Incident", updateSuccessCallback, errorHandler);
         }
    }
}

When page is saved, it displays the caseNumberId, but it alerts that record has not been saved, to stay on the page or click Cancel. If I try to save again, the caseNumberId is set to null.