No nothing is added to the config.json file. The above is implemented and tested on Ubuntu 14.04 and Ajent v1.2.23.8

Here is an example of the logs from a server:

Jul 28 00:55:23 localhost ajenti: failed login attempt for root ("fail") through AjentiSyncProvider from X.X.X.X
Jul 28 00:55:37 localhost ajenti: user root logged in through AjentiSyncProvider from X.X.X.X

1) create a new filter config file ajenti-auth.conf....

Step A: Run the below command

Sudo vi /etc/fail2ban/filter.d/ajenti-auth.conf

Step B: Copy the below and paste inside the above file
# Fail2Ban filter for ajenti
[INCLUDES]
before = common.conf

[Definition]

_daemon = ajenti

failregex = ^%(__prefix_line)sfailed login attempt for .* through .* from <HOST>\s*$

ignoreregex =

2) Add the jail settings to the end of the jail.local file...

Step A: Run the below command

sudo vi /etc/fail2ban/jail.local

Step B: Copy the below and paste inside the above file at the end
[ajenti-auth]
enabled = true
port = 8000
filter = ajenti-auth
logpath = /var/log/auth.log
maxretry = 3

3) Restart the service...
Step A: Run the below command

sudo service fail2ban restart