And diplomacy here could be the difference between a ticker-tape parade and the cold shoulder. Communication is key, but before you speak, consider a bad and better approach to getting your message across. She might put it on right there. That first approach makes her question her desirability. It sounds accusatory.
Nobody likes to feel guilt-tripped.
The second approach, meanwhile, is creative, flattering and playful. Trying a new position or adjusting a familiar one is less intimidating than insisting she fulfill that dominatrix fantasy you've been harboring. There is a difference between having an experimental attitude and making absolutist demands. Keep yourself and your partner free of demands, musts and shoulds. Train yourself not to take it so seriously.