>>5520887
You can give her your number on a napkin but don't actually verbally ask her out.
It's the job of Cashiers/CSR's to be nice to customers no matter what, and they also can't just walk away from you to end the conversation because they have to stay at their post. Asking her out verbally will make her feel trapped and put her on the spot (It puts everyone, everywhere on the spot) but worse because she can't respond naturally to it since she's at work.
You might be one of those dipshit guys who thinks cornering a woman and laying the pressure on her increases your odds of getting a positive answer, and somehow thats good to you even knowing she will likely back out after - but the opposite is true. You're far more likely to get a positive response from her if her first encounter with you isn't uncomfortable, aggressive and negative.
Just go in and order/buy something, ask her about her day and stuff, smile, try to be charming but for the love of god don't talk too much or you'll shoot yourself in the foot. Leave for a bit (if its a food place, eat your food and then when you're done go back up to her) and give her your number and just smile and walk out).