。
Tell me what you want.
Literal
Wish [of-の] thing [object-を] please-say.
望み ('wish, desire') is the masu-stem of 望む used as a noun. 望みのもの ('the thing of [one's] wish') = 'what one wants.' A generous or gift-giving offer.
。
Tell me what you want.
Tell me what you want.