Assuming you already have the dhcp pool setup.
For the sake of example, we'll say it's called "DHCPPOOL"
ip dhcp pool DHCPPOOL
option 66 ascii "http://whatever.you.want.here/"
If you have trouble with the device not picking up the option 66, it could be because of the quotes. Try removing them like:
ip dhcp pool DHCPPOOL
no option 66 ascii "http://whatever.you.want.here/"
option 66 ascii http://whatever.you.want.here/