How about this? The username holds the original username without the token, token is the 15 character token and uTok is the username with the token
def generator = { String alphabet, int n ->
new Random().with {
(1..n).collect { alphabet[ nextInt( alphabet.length() ) ] }.join()
}
}
def token = generator( (('A'..'Z')+('0'..'9')).join(), 15 )
def username = "JayKay"
def uTok = "${username}${token}"
println "==>${uTok}<=="