How to use 要想 and 要 and 想
Because 一定想要 is hard to be present alone. 一定想要 typically exist with other things. For example, 他人不一定想要, 這樣發朋友圈的女生一定想要男人, 他們到不一定想要馬上定下來或進入婚姻, and tones of example can be found with google with "一定想要" on search bar. These example shows that people use 一定想要 with other things together. It is hard to found 一定想要 exist along. Therefore, your friend can do it in his question but not in your reply. Your friends question have 你 before 想要. while your reply only have 一定想要 exist alone. You can remove 一定 so that you can use 想要 too.