Halakhah su Salmi 146:10

Kedusha is recited in the third berakha of Ḥazarat Ha-shatz. The essence of the Kedusha is the congregation’s response with the verses: “Kadosh, kadosh, kadosh, Hashem Tzevakot, melo kol ha-aretz kevodo” (“Holy, holy, holy is the Lord of hosts; the whole world is filled with His glory”; Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (“Blessed be the Lord’s glory from His place”), and “Yimlokh Hashem Le-olam…” (“The Lord shall reign forever…; Tehilim 146:10). The custom today, based on Arizal, is for the congregation to recite the connecting passages as well. The ḥazan then repeats them aloud, and the congregation responds with the verses of the Kedusha (MB 125:2; Kaf Ha-ḥayim 2).

He says “כתר²¹³The Keter, כתר, was an ancient beginning of the Kedushah, קדושה, or sanctification prayer. It is no longer said and Kedushah now begins with the word na'ariẓkha, נעריצך. It is part of the Sabbath and festival Musaf Service (see footnote 166).
The Kedushah said during the Musaf Amidah is different than the Sanctification recited in the Shaḥarit Amidah on the Sabbath (see footnote 17). The Musaf Sanctification dwells more sublimely on the majestic conception of the angels in heaven glorifying the Eternal King and it introduces Israel proclaiming in response to the angelic choir, the holiness, glory, unity, and sovereignity of God's name. The Shema, the congregation's confession of faith, is part of the Musaf Kedushah (see footnote 17, section (3)).
The Shema entered the Musaf Kedushah as the result of a sixth century persecution. The Byzantine Empire forbid Jews to recite the Shema in public worship. On Sabbaths and festivals government spies would enter the synagogues to make sure the Shema was not said in its regular place during the Shaḥarit Service. As a means of getting the Shema into public prayer, the reader would insert it into the Musaf Kedushah, a place where the spies would not expect it and thus not be aware of it. The Shema has remained a part of this Sanctification ever since.
The Kedushah is part of the third blessing of the Amidah and in the Musaf Service for the Sabbath it is made up of the following parts along with connecting sentences: Isaiah 6:3, Ezekiel 3:12, the Shema, Deuteronomy 6:5, and Psalms 146:10. On festivals there is an additional verse from Zechariah 14:9 following the Shema.
Hertz, op. cit., pp. 528-31; 816-19.”, “the crown”, just as in the Additional Service.²¹⁴Musaf, מוסף; see footnote 166.