This Halacha is an excerpt from our Sefer
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Appointing someone to count on one’s behalf:
One cannot appoint a messenger to count the days on his behalf. This applies whether one asks an individual to count on his behalf, or one asks a congregation, or Chazzan to count on one’s behalf.
 Admur ibid
 The source: This is learned from the verse Emor 23:16-17 “And you shall count for yourselves from the day after the Shabbos [which refers to the first day of Pesach], the day that you bring the Omer offering. The [counting] should be seven complete weeks” and the next verse says “[You are to count] until the day after the seventh week you are to count 50 days…” Now, the Sages expounded from this double wording of “Count for yourself” and “You are to count” that it is not valid for simply the Beis Din to perform the counting, as is done by other Mitzvos such as Yovel, in which Beis Din counts seven Shemitah cycles and then sanctifies the 50th year as Yovel, and rather each and every Jew must count. [Admur ibid]