TFTP server on it's own does not have any reason to exist.
And because of that, there's no such option in DHCP (it's like specifying HTTP server in DHCP. Unless you know what is it for, it's useless).
From what I've read, polycom can download their configuration from HTTP and FTP, not only TFTP.
They use DHCP options 160 and 66 for configuration URL, option type is String.
Opt.66 is only checked if there is no option 160 in DHCP.
Examples of such URL:
ftp://xxx.xxx.xxx.xxx
ftp://username:password@xxx.xxx.xxx.xxx
tftp://xxx.xxx.xxx.xxx
http://xxx.xxx.xxx.xxx
xxx.xxx.xxx.xxx
I've taken it from correspoding thread in polycom support forum.
Just in case, here is example how to add custom options in DHCP server of Windows Server 2012.
Generally, you click RMB in IPv4 node under DHCP in Server Configuration (Server Manager) tree, and select Set Predefined Options
. You also could set predefined options for ranges and reservations, whichever is more appropriate/convenient for you.