How do I mention this to my girlfriend?

just be like hey baby you wanna tie me up with duct tape? lmfao im kidding i dont really know how you could bring this up with her. Maybe make it into some kind of game that you play? haha all i have to say is that duct tape is going to hurt to pull off haha so maybe you might want to consider whether or not you really wanna do this or not before you stress over how to express it to her... btw this totally made me laugh.. idk if you were going for that but it's just a very interesting fantasy you have. i wish you the best of luck!