Changing a custom field when the field wasn't set before

When setting a custom field that wasn't set yet, we'll actually need to check whether the custom field exists and then get the field id to set it to the correct field id. It is a bit strange that this is needed as with the company or tag field this isn't needed. But it seems that Highrise can't map the custom field labels to the ids automatically and thus we'll have to manually check which id the custom field is.