Reserving of tickets using the Hokkaido 4-day Flexi JR Pass

For the Hokkaido 4-day Flexi JR Pass, will it be possible to reserve tickets in advance? For example, I wish to travel from Sapporo to Hakodate on Tuesday in the early morning, thus will like to reserve the train tickets the night before on Monday night, is this possible and will this still count as one day of usage on the pass?