KeystoneJS: How to set a field to receive randomly generated value?

I'm creating a model that I will use to authenticate users for API access, and I have a secret field where I want to store a Base64 encoded uuid/v4 generated value.

I went through the different field types and options, but still not seeing how I could achieve this.

Is there a way to hook in model instance creation, and set the value of my secret field ?