Since you don't specify, I'm guessing that you are using the open source version of RStudio Server. The open source version only supports local linux passwords.
If you want to integrate with other authentication mechanisms to achieve single sign-on, then you should investigate the professional version, RStudio Server Pro. The pro version has support for LDAP, and in version 1.4 also supports SAML and OIDC. See https://blog.rstudio.com/2020/11/16/rstudio-1-4-preview-server-pro/ for more information.