Making a 2016 HSA contribution before April cut-off

I did not put anything in my HSA last year other than what my employer gave, so it is not maxed out.  I want to make a contribution to my 2016 HSA before April 15 to zero my tax liability.  Do I need to wait for a form back from Fidelity where I have my HSA, or what screen can I put in that amount once I've sent the check?