MikroTik RouterOS v2.9 Reference Manual page 615

system note, you may upload a plain text file named sys-note.txt on the router's FTP server, or,
additionally, edit the settings in this menu
Property Description
note ( text ; default: "" ) - the note
show-at-login ( yes | no ; default: yes ) - whether to show system note on each login
Notes
If you want to enter or edit multiline system note, you may need to use embedded text editor:
/system note edit note
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