Login messages have a common theme.  Usually, they warn users of the dire consequences of using the system for inappropriate access.   Here are a couple links with some good starting points.

https://www.security-marathon.be/sample-log-on-warning-banner/

https://security.tennessee.edu/login-banners/

You can of course write your own.  If you were working for a company, you would most likely collaborate with HR on this.