How do I approach my girlfriend about couple swapping?

So I've had this fantasy of seeing my girlfriend with another guy and asked for opinions on here... I met this amazing married lady and she's been encouraging me to ask my girlfriend if she'd like to couple swap

Now I am a bit embarrassed to bring this up to my girlfriend but I think it'd be fun to try it, and this lady I met told me she was willing to comfort me when my girlfriend and her husband have fun together

So what are some ways I can get my girlfriend used to the idea? Should I just straight up ask her out of the blue or should I build up to it?

How do I approach my girlfriend about couple swapping?
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