D365 portal select lookup field value on form load

 If you want to select your lookup field on form load or other events you can use this script.

 $(document).ready(function(){  
   var requeststudentId = '{{ request.params.id }}';  
   if(requeststudentId)  
   {  
     var eventLookupfilteroption = "studentid eq guid'"+requeststudentId+"'";  
     var odataUrI = "https://yourpotal.com/_odata/students?$filter="+encodeURIComponent(eventLookupfilteroption);  
     $.ajax({  
       type:'GET',  
       contentType: 'application/json; charset=utf-8',  
       datatype: 'json',  
       url: odataUrI,  
       beforeSend: function(XMLHttpRequest){  
         XMLHttpRequest.setRequestHeader('Accept', 'application/json');  
       },  
       async: true,  
       success: function(data, textStatus, xhr){  
         if(data.value[0]) {  
                          // Populate lookup field  
           var studentid = data.value[0].studentid;  
           var studentid_name = data.value[0].proevt_name;  
           var studentid_entityname = "student";  
           $("#studentid").attr("value",studentid);  
           $("#studentid_name").attr("value",studentid_name);  
           $("#studentid_entityname").attr("value",studentid_entityname);  
                          // Populate other field  
           var eventCode = data.value[0].studentcode;  
           var studentstartdatetime = data.value[0].studentstartdatetime;  
           var studentenddatetime = data.value[0].studentenddatetime;  
           var proevt_freeregistrations = data.value[0].proevt_freeregistrations;  
           $('#studentcode').val(eventCode);  
           $('#studentstartdatetime_datepicker_description').val(studentstartdatetime);  
           $('#studentenddatetime_datepicker_description').val(studentenddatetime);  
           $('#proevt_freeregistrations').val(proevt_freeregistrations);  
         }   
       },  
       error: function (xhr, textStatus, errorThrown ){  
         alert(textStatus+ ''+errorThrown);  
       }  
     });  
   }  
 });