Join the waitlist in the app and make sure push notifications and marketing emails are switched on. Deals in the sandbox period are limited and have closed quickly, so notification settings are the difference between hearing about a campaign and reading about it afterwards.
There is no reservation or pre-purchase mechanism ahead of a launch. Allocation is on a first-come basis once a campaign opens, subject to the deal's own terms.
If you registered interest and received confirmation that an allocation is secured, that message will state the next step required from you.