So long as you have established your HSA prior to incurring the expense, you would be able to reimburse yourself later for that expense. So for example, if you were to establish your HSA tomorrow and funded it with $1 and then the very next day you got sick or injured, you could pay for those services out-of-pocket and then reimburse yourself with your HSA dollars whenever you had enough money in it at some point in the future.