May have to call in. It should work, but who knows with IT making changes.
You could try the chat feature, not sure if they could do it though.
If this is for your cruise this month, you should be fine for OB purchase.
You don’t have to worry about an increase for a June sailing.
Once OB, you can just go on the App and add it, or go to GS or the crew around the ship selling the beverage packages.
if you buy it ahead of time, you’ll pay extra for your insurance coverage. If you purchased insurance. If you didn’t, then doesn’t matter.