how to set up DHCP option 26 on server?

RouterOS Beginner Basics
1

I want to limit my windows7 dhcp clients interfaces to MTU 1480 instead of 1500
Is it possible? RFC 2132 say to use option 26 but how? I add string into IP - DHCP server - Options:

Name Code Value
MTU 26 5c8

and nothing change
still can ping 192.168.0.253 -f -l 1472