Your letter is being sent straight to the Ohel. It is private and will not be seen by Chabad Caldwells.
When asking the Rebbe for a blessing it is costumery for one to take a new resolution in the study of Torah or a special new Mitzvah to take on to be a "Vessel " for G-d's blessing.
Examples may be: Lighting Shabbat Candles, Giving Charity, Study Torah etc