You have to wait until she has had hers then slip it under her at night when it's dark as their eyesight isn't so good in half light. Chances are she would accept it then.
If you put it under her during the day, even if she has chicks of her own, she is very very likely to kill it (giving it a good peck) as it is a threat to her own getting food.
You have to be careful doing it, making sure the chick you are putting in isn't bigger etc.
Nowt's simple