Ok, finally after several test I make it working.
the correct syntax is:
dhcp-option=66,"192.168.1.4"
dhcp-boot=pxelinux.0,,192.168.1.4
so you only need to create a file named 10-TFTP.conf on /etc/dnsmasq.d/
and add this lines.
remember to change my 192.162.1.4 IP for your TFTP server IP, and be sure that you have the file pxelinux.0 on the root folder of your tftp server.
I finally find this: https://www.reddit.com/r/pihole/comments/5vshcz/how_to_use_the_pihole_dhcp_server_for_pxe_boot/
to guide me.
Thanks to developers & Moderator for his help.
Please consider to mark as SOLVED this case