How to create a custom field for contact?

[Kelve_Neto]

How can i create a custom field for contacts ?
I am trying this : DataServiceExtensions.AddCustomField(null,typeof(CustomFieldType.Type.Email), "_KiaiDayInviteEmail", 1);
But dont seem to work as it gives errors. Am i doing something wrong ?

[TomScott]

https://developer.infusionsoft.com/docs/rest/#!/Contact/createCustomFieldUsingPOST