How to define an optimizer with a maximization target in guild.yml?

I see some CLI syntax to define an optimizer with a maximization target, but I can’t find the way to define that in the guild.yml file.

Let’s say I have 2 input parameters, a and b and I want to do a search for a = [0.1, 10], b = [10, 1000] in order to maximize the scalar output parameter z.

How do I create a guild.yml file for that?