How do I ask to be spanked?

Me and my boyfriend are usually really open about our fantasies and kinks and lately we’ve been trying a lot of new things together and it’s going really well. This made me think that maybe we could try spanking. He slaps my ass during sex and casually just around the house when we’re not having sex but he’s never really bent me over his knee and spanked me like I want. I usually have no problem asking him to do things to me like tie me up or edge me etc. but I’m a little embarrassed about this kink in particular. How do I go about asking him? Preferably a way where I don’t just have to flat out say it although that may be the only option :/.