To create a banner to deliver pre-log-in messages to users on an NSM, below is the procedure –
1 – Login as super user.
2 – Select Administer and Login as the super user.
3 – Select Administer.
4 – Select Server Manager , then Servers.
5 – Select GUI Server or GUI Server Cluster (HA Mode).
6 – Right click and Edit the GUI Server / GUI Server Cluster:
There is a place one can enter the Banner message in Log in Warning Message.
7 – Select OK. The message will appear in the Banner.
Reference –
https://kb.juniper.net/InfoCenter/index?page=content&id=KB29464&actp=search