I think you can buy discounted tickets on-site.
Visit Fuji Q Highland, go to the ticket office located in front of the entranc of Fuji Q and show the web page.
You can buy tickets with discounted rate.
I found the discount offering web page is provided just in foreign languages, which means this might be secret to Loco Japanese customers
So you should let the page on the smartphone screen shown to ticket office staff with notice that " I have a discount page".