How do I generate a challenge for elliptic curve signing using NIST standards?
I believe you're misunderstanding how ECDSA works.

When you sign a message with RSA, you pad out the message somehow; some padding methods are determanistic, and some are randomized.  However, once you get this padded message, you present it to the underlying RSA private operation, which is entirely determanistic.

However, ECDSA isn't like that.  Instead, the ECDSA private operation selects a value k; this value k is not the message being signed, instead it's a random value (and there are known attacks that can occur if it's biased; that's why FIPS 186 goes into such detail about exactly how you do it - if you ignore their advice without knowing what you're doing, well, there be dragons here).

Now, once you have the value k, you can take the message (actually, the hash of the message), and then present that to the ECDSA signature operation to generate the signature.

So, with ECDSA, there is a hash involved; however it's the message that is hashed, not k.