asking mom to sleep at boyfriends ?

so i’m 20 years old i live at home. me and my boyfriend have been together for a year and for two months earlier in our relationship he lived w us due to family issues, my mom knows that we have sex. his parents are away for the night and i want to sleep over but i need to ask my mom because my car is out of commission right now and if need a ride. what can i say to get her to say yes ?