Just be quick mate and say something along the lines of:
"I have a note here for the Captain. It's a jumpseat request. Would you be able to pass it on if it's not too much trouble? Thanks".
Boarding time is a very busy period for the FAs so be quick and do whatever he/she asks you to do.
Unless it's a training flight or the Captain doesn't want the hassle, you should be in!
Good luck,
VFE.