On the bottom row of the icons above the reply box is a thing that looks like a picture of a mountain with a sun or else it looks like a postage stamp. It is the fifth icon from the left end of the bottom row. Click it!
Unlike the old forum it does not want an "image" code it wants the "url" code.
The little sucker is finicky and wants you to hit the "enter" key when you have the "url" code properly entered. Properly entered means not having two "http" do whackey dos.