From configure
edit system services
set system services dhcp pool 192.168.1.0/24 default-lease-time 3600
The above command should hold the MAC Addresses, of your client machines to their respected IP of the pool.
The 192.168.1.0/24 should be set to your desired pool, and the 3600 should be modded to match your desired lease time :)
Hope this helps! If you need further please let me know, I would be happy to help you get your SRX going!
