Creates a new custom property. The key field is used to identify the custom property, it's written in snake case and it must be unique within the custom object. The label field can be used to give a human readable name to the custom property. The type field is used to define the type of the custom property i.e. text, number, float boolean, date, list, map. The required field is a boolean used to define if the custom property is required or not. The default field is used to define the default value of the custom property. The default field is required if the custom property is required and its value should match the type defined in the type field. The references field is used to define the associations between the given custom property and custom properties of other custom objects. Setting a reference will not only update the references of the given custom property but also the references of the custom properties that is being referenced.