This is used to add a classless static route to the DHCP clients.
To add option 121 to a Mikrotik DHCP server, it's value is specified in HEX. The format is as follows.
where n=mask, d=destination, g=gateway.
To convert ip address to HEX, you convert each octet, so 192=C0, 168=A8, 55=37, 1=01
You can use a tool such as http://www.miniwebtool.com/ip-address-to-hex-converter/?ip=192.168.55.1
To add a route to the destination network of 192.168.55.0/24 via gateway 172.16.10.1.
/ip dhcp-server option add name=classlessroutes code=121 value=0x18C0A837AC100A01
where 18 is 24 in hex.
*note: depending on the subnet mask, you may only need to specify 0-4 octets. In fact only the non-zero, or network portion of the subnet. Here is a table from the RFC.
subnet mask Number of octets 0 0 1- 8 1 9-16 2 17-24 3 25-32 4
so for the /24 subnet in the above example, we only needed to specify 3 octets - C0A837