It's impossible to load custom add-in manifest.xml in Word

Hi Maxime CHARRIERE,

Thank you for posting in Microsoft Community.

Regarding to your description and based on my understanding, I'd like to suggest a few steps to load a custom Add-in from manifest.xml in Word desktop that is use a shared folder (network share) as an add-in catalog, it will allow Word to trust add-ins from that specific location.

Step 1: Host Your manifest.xml file on a network share

• Create a shared folder on your local network that all your colleagues can access (e.g., \YourServer\SharedAddins or a SharePoint document library folder synced to local drive).
• Place your manifest.xml file inside this shared folder.

*Note: While your manifest is hosted on GitHub Pages, for direct sideloading from Desktop, using a local network share is the common sideloading catalog approach. Alternatively, if your colleagues can directly access the GitHub Pages URL, they might need to use a slightly different approach as outlined in documentation on how to add a shared catalog via URL, but the network share is typically simpler for small groups.

Step 2: Configure Word desktop to trust the shared folder as an Add-in Catalog

*Please encourage each colleague who needs to use the add-in to do this step once on their Word Desktop application:

1. Open Word > File > Options.
2. In the dialog box, select Trust Center from the left pane.
3. Click Trust Center Settings... button.
4. In the Trust Center dialog box, select Trusted Add-in Catalogs from the left pane.
5. In the Catalog URL field, enter the network path to your shared folder where the manifest.xml file is located (e.g., \YourServer\SharedAddins).
6. Click Add catalog.
7. Check the Show in Menu checkbox next to the newly added catalog.
8. Click OK to close all dialog boxes then restart Word

Step 3: Insert the add-in in Word

1. Open Word > Go to the Insert tab on the Ribbon > Add-ins group > My Add-ins.
2. In the Office Add-ins dialog box > Shared folder (or My Organization if deployed via Centralized Deployment) tab at the top.
3. You should see your custom add-in listed there. Select it and click Add.

Important Notes:

• Office Add-ins vs. COM/VSTO Add-ins: This is different from older COM or VSTO add-ins, because they're have different installation methods.
• Centralized deployment (for Admins): If your organization has Microsoft 365, an administrator can centrally deploy the add-in to specific users or groups via the Microsoft 365 admin center. This makes the add-in appear directly under the Admin Managed tab in My Add-ins without users needing to configure Trusted Add-in Catalogs. This might be another option if you have an admin involved.

For more detailed information on sideloading Office Add-ins, you can refer to: Sideload Office Add-ins for testing from a network share - Office Add-ins | Microsoft Learn

I hope this helps. If you have any further questions or concern, please don't hesitate to let me know.

Best Regards,
Mia - MSFT | Microsoft Community Support Specialist.