If it works like the real thing, put MID/-20 in the scratch pad and enter it in the flight plan. Then use the Vertical Revision page for this point (which should come up as PD01) and enter a restriction of FL260. Selecting Constraint on the FCU will show F260 in magenta next to your point PD01 and the 'aircraft' will fly to make that point at that altitude when Managed Descent is selected.