How to get dall-e to leave negative space?

If I want to leave a negative space to add text, etc.? How can I do this? And how can I tell Dall-e how much and where to leave the negative space?

Example, “a man jumping for joy in the desert”
But I want the top 30% of the image to be just the desert background, and not have any part of the man in that part of the image?

That will likely take a mixture of prompting and photo editing techniques to achieve exactly what you want.

“white background” / “studio photo” / “marketing photo”

…should help for just white backgrounds…