How to book the resort shirakami train

Can I ask one question- maybe you can help me. I’m trying to book onto this train using my pass which I have already purchased to travel on Monday 26th February- but i can’t figure out how to book it. Any ideas how to book or who I can ask? I tried emailing the support but got nothing back. When I put the start and end points on the website it doesn’t show this as an option https://m.jrpass.com/blog/resort-shirakami-train