How can dhclient request a prefix delegation of a specific length from provider?

[John Ioannidis]

If I specify the -P option, my provider dutifully delegates me a /64.
It will, however, delegate me a /56 if I ask for one. How do I specify
the length of the prefix I want delegated?

If it's documented somewhere, I can't find it :(

Thanks,

/ji