This strategy is terrible. I understand that it has worked for you before but even a broken clock is right twice a day.
The conversation has just started so she hasn't invested anything into you yet. Once she is invested you can mention FWB or the sexual arrangement otherwise you will scare her off. Although its less risky to imply it without saying it.
Your conversation is bland and boring. Add some humour, some joking, some teasing, some slight sexual innuendos etc. Throughout the dialogue keep the conversation fun, engaging and non-serious. If you don't come across as serious she will likely put you into the "non commitment" pile rather than the "relationship pile" and assume you are looking for something casual, which you are. The long and short is you don't have to verbally say you want FWB for her to work out your intention!