As it says in the description, you need to spike the bottle of wine in the kitchen before confronting Najma in her room. If you can persuade her not to eat you, and then confess to your plan and tell her where you put the poison, she'll get angry and you'll get tinier...
To be a little more specific, this means first climbing the kitchen island and putting the drug in the wine bottle there. Then you have to go to Najma's room and wait for Bayley to leave before going in and waiting for Najma to go to her bed. There are various ways to get through the conversation with her, but the quickest I think is to go under the other bed and wait, so she moves, then climb the desk and see the computer and her posters. Then if you go and confront her and bring up the books and her writing, then ask to move on and tell her about your plan. Tell her where you put the drugs, and the DLC should begin.