You can replace the default self-signed certificate for servermgrd with a signed certificate using Keychain Access. When you assign a trusted signed certificate to servermgrd and remotely connect to your server, the certificate warning no longer displays.
Import your certificate identity using the Server app.
For information about importing a certificate, see Import a certificate identity.
Open the Keychain Access app (located in /Applications/Utilities/).
If keychains are hidden, choose View > Show Keychains.
In the search field (located in the top-right corner), enter com.apple.servermgrd.
Double-click the com.apple.servermgrd (identity preference) system keychain.
Click the Preferred Certificate pop-up menu, and choose the imported trusted certificate.
Click Save Changes.
If prompted, enter your user name and password, then click Modify Keychain.