I'm going to drop the soap on purpose when I shower with my girlfriend
Why would you want to to drop the soap when you shower with your girlfriend?
I want her to bend over to pick it up
You should be more honest with your girlfriend if you want to have sex with her. You should not manipulate her or the situation by dropping the soap because that is dishonest behavior.
I just want a blow job. It's not a big deal.
Then you should ask her for one. Why lie about it?