To enable password expiry and a warning for password expiry, please apply the following lines in IdentityServer.exe.config within the appSettings tags
<!-- Warning period and expiration of passwords -->
<add key="Login_Password_WarningDays" value="360" />
<add key="Login_Password_ExpiredDays" value="365" />
In the above lines, a Password Expiry warning is set for 360 days, and if password not changed on seeing the warning, its set to expire on the 365th day.
IIS Reset (if EUM is installed On Premises) or App Service Restart (if EUM is installed as App Service) is needed after the above changes are done.
Note: Password expiry will not work if EUM is tied up with Azure AD