In Russian, хотенье (khotenie), "will" (our translation), is the noun from хотеть, "to want", and the book distinguishes it from желание, "desire", and from воля, "volition". The narrator asks for his own will, willful and free, which fits into no classification, and says that will is the manifestation of all of human life, while reason is only reason.
Where It Appears
Quotes
His own will, willful and free, his own caprice, however wild it may be, his fantasy, irritated at times even to madness: all of this is precisely that omitted advantage, the most advantageous of advantages, which fits into no classification and because of which all systems and theories shatter and go to the devil.
Part One: The Underground, the underground man
reason is only reason and satisfies only the rational faculty of man, while will is the manifestation of all of life, that is, of all human life, with reason and with all its itches. And although our life, in that manifestation, often ends up as some little mess, it is life, and not merely the extraction of a square root.