The Arizal and his followers instituted the practice of adding "Hin'ni Muchan Umzuman" before many different mitzvot, with the specific text changing to suit the mitzva. Examples include introductions to Friday night Kiddush, to Birkat Hamazon, to each of the four cups on Seder night, and of course to S'firat Haomer.
For whatever reason, some of these liturgical additions have found more popularity than others, and none more so than S'firat Haomer. This is somewhat ironic, as the text explicitly says "I am prepared to fulfil the commandment as is written in the Torah", while according to most commentators, the mitzva of counting the Omer is only considered to be a mitzva d'orayta while the Temple stands. Nevertheless, the custom to recite this paragraph before counting S'fira is extremely prevalent.
Samuel Alman's setting for S'firat Haomer is one of his most beloved compositions. Alman makes wonderful use of both chazzan and choir with some incredibly dramatic sections as well as some tender moments. Perplexingly, the final line before the b'racha (a reference from Psalm 90) is actually misquoted in Alman's published score. Historically, these words were never appended in Nusach Anglia (finishing at "mincha chadasha ladonai"), so perhaps Alman misremembered the quotation when he composed the piece, or perhaps there was some publishing error. Some arrangers have attempted to correct this by duplicating the musical phrase with the missing stitch, but this loses something of Alman's succinct drama. Instead, I have reset the words to best fit Alman's original music.