How to store private key secretly in server such that is could be used by my web application only?

So, I am developing an application, where I need to store private key to decrypt the encrypted text on server side.

How can I protect my the private key in the server ?
My director is asking me to store somewhere in vault, but even vault need authentication. And for authentication u need token, now question again arises where to save token ?

application asking cybersecurity decrypt director encrypted key private private key protect server server side store text web web application