This is from my own NetScaler wiki:
Rewrite policies to add text on logon page - X1 theme (not RFWebUI)
add rewrite action act_rw_Logon_Text insert_before_all "HTTP.RES.BODY(120000).SET_TEXT_MODE(IGNORECASE)" q{"var login_footer=$(\"<div style='background-color:rgba(86,86,86,0.6);text-align
center;colorwhite;font-size:15px;'><br>Logon with your current username and password,"+" then change your password when prompted<br><br></div>\").appendTo(logonbox_container);"} -pattern "box_view.prepare_view();"
add rewrite policy pol_rw_Logon_Text "HTTP.REQ.URL.PATH.EQ(\"/vpn/js/gateway_login_view.js\")&&HTTP.REQ.HOSTNAME.SET_TEXT_MODE(IGNORECASE).EQ(\"
my.fqdn.com.au\")" act_rw_Logon_Text