When Fusion starts up, it looks in four locations for add-ins and will automatically load them at start-up.
- Default Path that is specified in the API preferences, as shown below. The example below shows the default path, which is user-specific, but the user can edit this to be any location. Because this isn't consistent, it's probably not the best location to install an add-in.

A user-specific folder. This location is consistent but is meant to be where add-ins from the App Store are installed. However, nothing is preventing you from installing your add-in here. One thing that's strange about add-ins installed here is that they are expected to be in a .bundle folder. This folder on my Windows computer is: "C:\Users\brian\AppData\Roaming\Autodesk\ApplicationPlugins". There's an equivalent user-specific folder on Mac.
An add-in called "MyTestAddIn" will need to have a structure like this:
MyTestAddIn.bundle (This is a folder with your add-in name and appended with ".bundle")
PackageContents.xml (This file contains information about an App Store app.)
Contents (This is a folder.)
(Your add-in Files. This is the contents of the "MyTestAddIn" of the original add-in.)
I'm not sure if the PackageContents.xml file is required in the case where the add-in is not coming from the App Store. I would try it without it first and if Fusion doesn't see the add-in, add an empty file; if it still doesn't see it, copy the file from an existing add-in from the store and edit it to represent your add-in.
There's a machine-specific folder that on Windows is "C:\Program Files\Autodesk\ApplicationPlugins" and there is an equivalent folder on Mac. This folder is rarely used and I'm not sure if it requires a .bundle folder or not but some quick testing can tell you.
A specified location. The final location is defined when you use the green "+" at the top of the "Scripts and Add-Ins" dialog. When you do this you can browse to any location on the machine and choose the add-in folder. This is unique for each add-in and Fusion remembers it so the next time Fusion starts up it will go through the list of add-ins that were added in this way and load each one.
There isn't a way to add an add-in to this list outside of using the "Scripts and Add-Ins" dialog, so this isn't a viable option for installing a new add-in.
I would suggest using option 2. You'll have to do a bit of experimenting to see what's required, but it won't be hard.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com