If you have experienced your download restarting from scratch, I strongly suggest you use this simple guide:
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1. Make sure the both PCs have the Windows 10 Anniversary Update (build 1607) installed
2. Download and install Fiddler. This program will be used to find the link to the install package generated by the Windows Store.
3. In Fiddler, click WinConfig, and in the following menu check Store. Save Changes afterwards and close the WinConfig window.
4. In the Windows Store, start the download for Halo 5: Forge. If it doesn't work there, or you do not want the Halo app as well, install from the My Library option when you click on your account image
5. In Fiddler, check the traffic for a large amount of requests to a Microsoft website. Right click one and click Copy -> Just URL. If you do not know what to look for, see this example (basically, a URL containing '/filestreamingservice/files').
6. After copying the link, close Fiddler, cancel the download in the Store and paste the link into your web browser or a download manage and let it complete. So, basically you should see this (if we were downloading Forza 6 Apex).
7. When the download is complete and you're back on your target PC (with the downloaded .Appx/.EAppx). Start the download of Halo 5 Forge from the Windows Store on this PC, but pause it after about 10 MB. This is so your PC obtains the licence information for the game so it can activate on first launch. After that is done, pause the download and close the Store
8. Open the folder the install package is located in Windows Explorer, press Alt + D and type 'powershell' and hit Enter.
9. In the Powershell window type the following (hint: rename the install package to something short beforehand) Then just wait for the install to finish (If you want to install somewhere other than C: drive, go to Settings -> System -> Storage and change the install drive under 'New apps will save to:')
add-appxpackage Halo5Forge.EAppx
10. Launch Halo 5 Forge from the Start Menu. Provided your PC meets the requirements, it should work.
Hopefully this helps some people get around the closed nature of the Windows Store."