We do not offer Widevine in PureBrowser or in our repos. It is closed source, DRM and thus goes against our dedication to freedom, privacy, and security.
You would need to install Firefox or Chrome to watch Netflix in Linux. This is not something the PureOS team recommends, due to inherent security risk of closed source software, or would be able to help out with. You’ll need to turn to the community for help on this one. There may even be a post in the forum on how to do it.