iRule to replace NetBIOS-Name with sAMAccountName

According to the wiki page for HTTP::username, it's a read-only command, so you can't update the username with that command. Here's spin off of what I've done in the past that may work for you:

when HTTP_REQUEST {
    if { [HTTP::header exists "Authorization"] && [HTTP::header value "Authorization"] starts_with "Basic " } {
         Basic Authentication
        log local0. "  Authorization: [HTTP::header value "Authorization"]"
        log local0. "    Username: [HTTP::username]"
        log local0. "    Password: [HTTP::password]"

         Parse the username
        set user [getfield [string tolower [HTTP::username]] "\" 2]

         Recreate the Authorization header
        set newAuth "${user}:[HTTP::password]"
        set newAuthEnc [b64encode $newAuth]
        HTTP::header replace "Authorization" "Basic $newAuthEnc"

         Remove the variables
        unset newAuth
        unset newAuthEnc
        unset user
    }
}

NOTE: I saw you were using

"/"

getfield

"\"