Wow. That's pretty intense to just say, "Here, go try on a strap on and see if she likes it."

Strap-ons can be really intimidating for both you and the person wearing it.

Even though it's one of my fantasies, I think I'd be feeling awkward at first to stand in front of my partner or even the mirror with a dildo dangling between my legs, and I think it'd be awkward at first to for my partner to see me with a dildo on.

I'd try something else first, perhaps her using blindfolds on her, having her on top of you giving orders, having her feed you, etc. Start with the basics of dominance, I just think strap ons can be intense, but you can bring it up and see if she is into it.

But if you do go with the strap-on, be sure to vocalize what you are and aren't comfortable with regarding the dildo and the way she uses it. You may want to start with a nude color or even a non-realistic color like green or color depending on what would make you feel comfortable. I would start at 4 or 5 inches, you don't want to go too big too fast. You would have to take her to pick out a harness, as harnesses come in all shapes, material, and sizes. Then I would discuss how she should use it. Personally, I'd start with oral first to get used to it before trying an*l. Be sure you pick up specifically an*l lube, not just regular lube. You also might want to pick up a small butt plug to wear right before you use the strap-on to get you physically prepped for it.