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Is there a way to at least make an external suitelet secure or use some sort of authentication? I know it wasn't designed to have auth, but any ideas on how this can be implemented?

I think you could try NetSuite webpage authentication

and try to embed or try using OKTA of they have authentication strategy

Or use an API key type-thing as a GET parameter 

the use case is the 3rd party system will only have the ability to put a link in their end. Thus a link to an external suitelet will be provided. I was thinking of putting some sort of security. i.e. username/password, auth keys, or whitelist domain...

No, not oauth. Just whack a &amp;key=verylongstringrandomlygenerated on the URL, then check that it’s correct in your suitelet. It’s a ghetto way of doing it, but it’s simple &amp; works 