Convert FQDN to netbios domain name

Look at this link https://devcentral.f5.com/codeshare/apm-variable-assign-examples-1107

You can create a variable assign for variable

session.logon.last.domain

switch [string tolower [mcget {session.logon.last.domain}]] { 
    "test.contoso.com" { return "test" } 
    "test2.contoso.com" { return "test2" } 
    default { return "test3" } 
}