The following variables are usable inside of the DHCP-Server script tab.

• leaseBound – set to “1” if bound, otherwise set to “0”
• leaseServerName – dhcp server name
• leaseActMAC – active mac address
• leaseActIP – active IP address
• lease-hostname – client hostname
• lease-options – array of received options

The DHCP server script runs every time there is a new lease, or a lease expires. Doesn’t look to run when static leases renew.

https://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Server#General