Mag ik een

"May I have a" is incomplete, leading to unspecific and random description which could mean a myriad of things. It is impossible to interpret this without additional information and context. Therefore, the phrase is not possible to be used as it is for image generation, and we need to use our imagination or specify an item that would fit into this context. A more specific phrase might be, "May I have a slice of chocolate cake on a white ceramic plate placed on a dark wooden table."