If you want to use DALL-E, the default prompt will be the “based on” term you set in settings (default: post title).
If you want to give DALL-E a custom prompt. There is an easy way to do this, see this example:
For example, if your post title is “Dog”, the custom prompt will be “Classic Art with Dog” and DALL-E will provide this type of image:
DALL-E will rewrite the prompt you entered and if you enable logs, you can see what the rewritten prompt is: