This is from my internal wiki I keep, it works on the latest 12.1 with RFWebUI:
Logon page text - RFWebUI
For Gateway vServer:
add rewrite action rw_act_LoginFooter insert_after_all "HTTP.RES.BODY(120000).SET_TEXT_MODE(IGNORECASE)" q{"(\"<div style='text-align
center;color#003c69;font-size:15px;'><br>This system is for use by staff of COMPANY "+"and approved contractors and in accordance with the COMPANY Computer policy</div"} -search "text(\"customAuthBottom\")"
add rewrite policy rw_pol_LoginFooter "HTTP.REQ.URL.CONTAINS(\"/LogonPoint/index.html\")" rw_act_LoginFooter
Then bind to gateway vServer
For AAA server, you cannot bind to AAA server, so check in the policy for AAA vServer hostname then bind gobally as below:
add rewrite policy rw_pol_RMS_LoginFooter "HTTP.REQ.URL.CONTAINS(\"/LogonPoint/index.html\") && HTTP.REQ.HOSTNAME.SET_TEXT_MODE(IGNORECASE).EQ(\"
fqdn.company.net.au\")" rw_act_LoginFooter
bind rewrite global rw_pol_RMS_LoginFooter 100 END -type RES_OVERRIDE