There are two different login messages. 

“login message” appears before you are prompted for your password. Example:

set system login message "Go away before you log in!"

“login announcement” is presented after you successfully enter your password. Example:

set system login announcement "you logged in!"

in either you can add a \n to start a new line or a \t to tab over

example- if you set this:

set system login message "\n\t\t\tSystem Managed by NetOps\n\n\tWARNING: Unauthorized access to this system is forbidden and will be\n\tprosecuted by law. By accessing this system, you agree that your actions\n\tmay be monitored if unauthorized usage is suspected.\n\n\t\t\tSo There!\n"

you will see this

			System Managed by NetOps

	WARNING: Unauthorized access to this system is forbidden and will be
	prosecuted by law. By accessing this system, you agree that your actions
	may be monitored if unauthorized usage is suspected.

 			So There!