You don’t really negotiate a one-pay. You decide after everything is optimized** how to structure the payments.
** Some dealers won’t do one-pays.
One-Pay Leases | General Questions and Discussion - Off-Ramp - Leasehackr Forum
These are almost always horrible. There is no substitute for fully researching what to pay. Not all terms make sense either (sometimes 24 months is the only reasonable choice, other times it’s 18 or 48 or 36 or 39 or 27 or… well, you get it 
).