The Prayers and Benedictions:
It would seem, furthermore, that when the Temple stood and it was the custom to give the Half-Shekel, they used to recite the benediction: Who has sanctified us with His commandments and commanded us to give the Half-Shekel. For this is said in B'reishit Rabba: The Holy One, Blessed be He, said: You sold Dinah, daughter of Rachel, for five s'laim, therefore everyone owes a beqa per head. In other words, there were ten who sold her for five s'laim; thus the portion of each is a Half-Shekel.
The Treasury of Commandment Benedictions
This applies too to the Half-Shekel according to Rabbi Yehuda bar Yaqar, insofar as they used to recite a benediction while giving the Half-Shekel. Its source is from the Midrash, i.e., for the sacrifices to bring to the Temple in order to atone for ourselves.
Positive Injunction # 171 - the Half-Shekel* - dependent upon others  (similarly RIBaP).
The Benediction Recited over Giving the Half-Shekel: it is written in the book Zikhron Eleazar ... not to recite any benediction over the giving of the Half-Shekel, because this commandment comes about merely because of danger, as it is written: lest there be a plague. The book Shem miShimon (section 28) disagrees, saying that this verse does not relate to the public sacrifices offered up annually, but rather to the Half-Shekel one gives while the census is going on for the purposes of the census, and adds that at any rate one should not recite a benediction over the giving of the Half-Shekel because this is merely a hekhsher mitzva, in preparation for the performance of a commandment, for the real purpose of the giving is merely to buy sacrificial animals and other public needs, and so no benediction is to be recited, as in the case of qiddushin.
Now with regard to the opinion of the Hacham Zvi (responsum 121), who wrote that when they used to seperate the Half-Shekels, they used to recite a benediction because they did nothing less than the commandment of challat chul (seperating the dough outside the Land of Israel). Accordingly, we may conclude that he did less (than challat chul) because this (seperating the dough) was the purpose of the commandment and it was completed, which does not apply to t'rumath ha-lishka, which is merely taking from it for public sacrifices [sources].
* When the Temple stood and they gave the Half-Shekel, they used to recite a benediction - Who sanctified us with His commandments and commanded us to give the Half-Shekel, as said in B'reishit Rabba (The Prayers and Benedictions: An Explanation, by Rabbeinu Yehuda son of Rabbi Yaqar).
 The author of this book made a mistake in including this
commandment in the category of commandments dependent on others,
taken from the Rashba. [Editor.]
Book of Responsa
In the Yerushalmi, Shekalim, Chapter Three, concerning the section reading: It is learned: they talked with him from the time he entered until the time he left, but he remained silent, Rabbi Tanhuma said: because of the benediction, and this we explained in Y'fe Mar'eh, "lest he swallow some of the water, in which case he would have sinned, by getting enjoyment without pronouncing a benediction. This seems to me to be unreasonable, for he could have uttered the benediction before, over some other [food], and intended [it to apply] to the water he would swallow. Moreover, we learn in the Mishnah: One who drinks water because he is thirsty recites the sheha-kol benediction. The Gemara explains that this teaches us of the exception, one who chokes over a bit of meat, for one who drinks water not because he is thirsty but because he is forced to do so is not to pronounce any benediction over the water, because he does not enjoy it, nor does it nourish him - a simple case. This applies even more to one who has no intention whatever of swallowing the water - he is certainly exempt from the obligation of reciting a benediction. This is so also where one is not to be prevented from filling his mouth with water because of the doubt as to whether or not he will swallow it. I think it is clear that the Yerushalmi means that one cannot remain silent because of the benediction one is supposed to recite when donating to the lishka while he is making the donation, for even if we say that this donation that he makes is mid'rabbanan and not mid'oraitha, nevertheless it is no less than challat chul, where we make a benediction when we make the seperation - and this is so with all the other mitzvoth mid'rabbanan.