EDIT: 2020/03/26: OnePlus explains the story behind The Lab in this thread: https://forums.oneplus.com/threads/the-stories-behind-the-lab.1204828/.
"The Lab" is a unique concept OnePlus first introduced in May 2016 with the OnePlus 3. The OnePlus 6, 6T, and 7-series were part of the programme.
The 7T-series' had it slightly different - participants had to buy the device first, and winners could choose between a full reimbursement of their purchase, or a custom device with their name engraved. This year's iteration (for the 8-series) is similar to previous ones - if you're selected, OnePlus will ship the device to you, free-of-charge, before the launch event. If you win the review competition, you get to keep the phone. Otherwise you'll have 1 month to ship the device back to OnePlus.
In total, only 10 reviewers will be selected, so make sure you make your application stand out. Try adding links to your existing work, if you have any - photography, news articles, blog posts, YouTube videos, etc).
If you're looking for some inspiration, check out previous reviews by The Lab reviewers:
Fun fact: AnonymousWP, an Oxygen Updater team member (and a mod of the r/OnePlus Discord server), is a OnePlus 7 Pro Lab winner. Check out his review as well.
Keep in mind, however, reviewers will have to sign an NDA before they receive the phones. Check out the official post on OnePlus forums, which contains more information, and a sign-up link as well.
If you're rather proceed to the sign-up form directly, click here. You'll have to answer a few questions, including a short write-up (about 250 words).
Applications opened yesterday, and will remain open until 5PM HKT, April 1st 2020. Selected reviewers will be announced on April 2nd 9PM HKT.
Best of luck! If you'd like to know more about the upcoming phones, check out our article (you can read it within the app as well, it's titled "OnePlus 8-series rumour round-up").
Source: OnePlus Forums