It works for me if the add-in is set to "Run on Startup" in the "Scripts and Add-Ins" dialog. If I don't have it set to "Run on Startup" then the setting is not remembered. However, if I disable the "Run on Startup" and then restart Fusion, the command is no longer promoted. I think that's OK because typically you would install the add-in on someone's computer with the default of "Load on Startup" defined in the manifest file. Unless they specifically disable it, the add-in will always be loaded and the promoted setting will be honored.
Here's the difference between isPromoted and isPromotedByDefault. The isPromoted provides exactly the same behavior as "Pin to Toolbar" option in the user interface. It promotes it to be displayed in the main toolbar. However, if the user runs one of the reset commands by right-clicking within the toolbar, Fusion will reset the specified panel or the entire toolbar. Resetting restores the layout to default. By setting the isPromotedByDefault property to True, your command will remain promoted even after the reset.
While I was looking at the isPromoted, I noticed a bug. I had set the add-in to not load on start-up and then once Fusion was running, I manually loaded it. It's not promoted, as I described above. However, when I went to promote, the "Pin to Toolbar" option in the pop-up is disabled. I believe it thinks it's already enabled. So it appears that if you load an add-in manually and any of the commands are set to be promoted, the promotion doesn't happen. It only works when the add-in is loaded at startup. I think this is a bug, but I don't think it's critical since that would not be a common workflow.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com