X plan, leasing, buying.....doesn't matter. That's just the price and how you pay for it. You can order the vehicle you want whether you're leasing it or buying it with or without X plan.
Just tell the dealer you have X plan and what you want to do. If they offer better than X plan make sure you get it in writing with all fees, taxes, rebates, doc fees, add-on options, etc. included for an out the door price and compare it to the same out the door price on X plan. Otherwise you run the risk of getting a worse deal because of hidden fees or other tricks.
Considering the dealer gets an extra $400 or so from Ford for the X plan sale it doesn't make sense that a dealer would turn that down if you're offering to use X plan unless they're planning to make it up somewhere else in the deal (or on the trade-in). Either way just get all the numbers in writing.