@SantushtS 

Why not use after insert business rule on sc_req_item table and handle this and it will work for all RITMs whose catalog item has that variable?

Ensure choice values are same between the variable and field on RITM

(function executeRule(current, previous /*null when async*/) {

	// Add your code here
	current.fieldName = current.variables.variableName;
	current.setWorkflow(false);
	current.update();

})(current, previous);

If my response helped please mark it correct and close the thread so that it benefits future readers.