Want to add to price a user-defined amount

Client wants users to be able to enter their desired additional donation amount on top of membership amount (pre-defined). Is there a way to reference the user’s entered amount within the Amount added field in the pricing rules?

I see that the number field could be set to steps. I don’t think they will want the users to have to page through 100 options to get the number they want.

The page I need help with: [log in to see the link]