Honestly, I'd be worried that they'd just say "cool, that's nice, good for you."
Probably better to be 100% explicit: "I am invoking my right to speak to an attorney" (as opposed to just saying you want to do something)
This is the advice I've heard before. Explicitly state you're exercising your right to remain silent and your right to speak to an attorney. There was some sort of precedence for it being important in a particular instance though I can't recall what the story behind it was.
Honestly, I'd be worried that they'd just say "cool, that's nice, good for you." Probably better to be 100% explicit: "I am invoking my right to speak to an attorney" (as opposed to just saying you want to do something)