Hi, Is it a Mitzvah for girls to serve the IDF? Where is the source for this?

ב"ה Shalom I will relate to your question from theoretical aspect and the practical aspect. From the theoretical perspective, the Talmud, (Kiddushin 2b) it states that it is not the way of a woman to wage war. This is a general statement without much clarification. However, the Talmud elsewhere (Nazir 59a) states clearly that a woman is not allowed to carry weapons and this is codified in the Shulchan Aruch (Yoreh Deah 182: 5). The question arises from a contradictory source in the Mishna (Sotah 44b) , which states that in time of "Milchemet Mitzvah" = a war which constitutes a Mizvah (such as saving Jews from enemies) "everyone goes out (to wage war) even a groom from his room and a bride from the Chupa." Several explanations were offered. The Radbaz in his commentary on the Rambam (Hilchot Melachim 7:4) gives two explanations . One is that since the groom is mobilized for war, the inevitable result is that the bride has nothing more to do under the Chupa, but it is not mmeant that she goes to wage war. In his second explanation he says, what is meant by the Mishna is perhaps that the women were involved in logistical things such as supplying food and water for the men who were waging war but not that the women actually participated in battle. (Similarly writes the Rashash on Sotah ibid) . So in the theoretical sense, the most a woman would do is logistical things as part of the war effort. However, from the practical perspective, with all our gratitude to Hashem with the establishment of the State of Israel and of Tzahal=IDF, since the inception of the State of Israel, it has always been the decision of the Chief Rabbinate of Israel, that it is forbidden for women to enlist in the army. ) See Techumin vol. 29 pg. 462). This is also the opinion of Rav Eliezer Valdenberg zt"l in Hilchot Medina (Vol. II, Sha'ar 3, Chapter 6). All the best