Here's a basic example
lua Code:
local frame = CreateFrame("Frame", nil, UIParent)
frame:SetHeight(200)
frame:SetWidth(400)
frame:SetPoint("CENTER")
-- Instead of using a backdrop, you create a texture as the background
frame.bg = frame:CreateTexture(nil, "BACKGROUND")
frame.bg:SetAllPoints(frame)
frame.bg:SetTexture("Interface/Tooltips/UI-Tooltip-Background")
frame.bg:SetGradientAlpha("VERTICAL", 0, 0, 0, 0, 0, 0, 0, .6)
Looks like this (it's pinkish because of the color of the skybox):
