To participate, click "Join Now" on the campaign details page. Once you've joined, submit your content URL through the mission progress page. Your content will go through a verification process, and once approved, it counts toward your quest conditions.
As you complete each quest, a "Claim Reward" button becomes available — click it to receive your reward. Rewards may include redeem codes, game keys, in-game items, earnings (cash), or physical rewards, depending on the campaign. You can check all received rewards in My Profile > Rewards.
When a campaign ends, any unclaimed rewards for completed quests are automatically processed.