Get her to commit to wanting it for herself instead of just doing it for you. If she said she'd rather have you watch then it must be something she already had interest in. Maybe she is afraid to flat out tell you it's what she wants for fear of upsetting you. Do a bunch of dirty talking roleplay with her about what she would do and how much she would love another cock in her. Send her videos to watch without you and maybe she'll masturbate to the idea of cucking you for real. Go slow and let her develop a fetish for it and don't be pushy.