How do I link "Travel Request" and "Travel Expense"?

How do I link "Travel Request" and "Travel Expense"?

When approving travel expense, is there a easier way to find which travel request the expense is related to?


One option I can think of is to create a travel number in Travel Request, but I'm not sure how to add a field for auto-generated sequential travel number in Travel Request form.  


THank you in advance