When you place the order, you can leave a note saying you want the external antenna option. So instead of the standard ESP-12F, the controller will come with ESP-07S WiFi chip, which looks like this:
https://www.mouser.com/datasheet/2/975/1494602250ESP-07S_Datasheet-1589078.pdf
and you can plug in an external antenna. There is no surcharge, so all you need is to leave a note. There will be a one-week lead time as we don’t normally stock these versions.