KnockoutValidation and the conditional required rule

I am trying to use KnockoutValidation with conditional statements. See code below:

self.transactionType = ko.observable('Option1');

self.ConditionalField = ko.observable().extend({
  required: true, 
  onlyIf: self.transactionType = ="Option2"
});

Unfortunately this doesn't work. I want to have ConditionalField only required if transactionType has value 'Option2'.

What is the best way to use conditional validation with knockout.validation.js?