This site is being deprecated.
Please see the official X‑Plane Support page for help.
I am not from Laminar Research; just a flight simmer from downunder.
You might try using Planemaker. It has been supplied with your purchase of X-Plane and you may have unknowingly loaded the software onto your PC.
I don't use it but I understand it has been made available for users to modify aircraft or build their own aircraft to use with X-Plane.
The on-line users desktop manual can be found at https://developer.x-plane.com/manuals/planemaker/
You may also search through Dr Google to find a 737-800 cargo transporter or have a look at the following site which is dedicated for X-Plane users found at https://store.x-plane.org/
When searching this latest reference you might look at the download section whereby flight simmers have created/developed aircraft just for X-Plane. Generally it is available as freeware but sometimes is available as donationware.
This site is no longer being actively maintained.