There is no official or legal "free license key" link for Clean Master Pro. Clean Master for PC is primarily distributed as

Using cracked software violates the Digital Millennium Copyright Act (DMCA) in the US and similar laws globally. While Cheetah Mobile rarely sues individual users, your Internet Service Provider (ISP) can see you visiting torrent sites. You risk fines or terminated service.

If you have already purchased a legitimate license and lost it, you should avoid unofficial "key generator" links. Instead, use these official channels to recover or manage your subscription: