A simple app to attemt to randomly "guess" an invite like this: https://account.oneplus.net/invite/claim/KFBS-IBDL-3PQD-FCDH for a OnePlus phone.
A single thread can handle about 6 invites per second.
Keep in mind that chances of actually getting an invite are miniscule (there are 36^16 combinations which is a huge number, even in computing)
Also, keep in mind that there are certain violations ethics behind claiming something that is not yours (and a certain romanticism, alas).
https://forums.oneplus.net/threads/a-scientific-approach-to-invites.72513/#post-3427373