TL;DR - By default, a custom functions add-in registered with Excel is available, but not running, when Excel loads. It only gets run if one of its custom functions is used (or if it has a task pane and the user opens it). Is there any way to auto-run it or trigger running it from a different add-in (in the same domain) that is running?
Details:
We have multiple custom functions add-ins: a "main" one everyone has and a few other optional ones providing sets of functions appropriate to different job roles, controlled by Active Directory membership. They use the shared runtime. Each add-in has a task pane (though the users rarely open them).
Sometimes, code in a custom function in one of the non-"main" add-ins needs to show a warning message to the user (not as its return value). Since they have task panes, we could have functions in each add-in show warnings by showing their task pane (showAsTaskpane) and putting the messages there. But there are cross-cutting warnings we would prefer to only show in the "main" add-in's task pane rather than having them show up willy-nilly here and there. That's not a problem when the "main" add-in is loaded in memory and running; we can use a BroadcastChannel to send messages to it. But if it happens that none of the functions in that "main" add-in have been used (yet) and the user hasn't opened its task pane, that add-in isn't loaded/running, so we can't send messages to it.
Is there any way to get that add-in running in that situation? Whether by auto-running it on load (I don't see anything in the manifest reference for that), or by having the custom function in the other add-in dynamically tell Excel to run the "main" add-in?
For your question: "Is there any way to auto-run it or trigger running it from a different add-in (in the same domain) that is running?" I think the answer is no.
Not sure whether helpful, I'd like to provide you with a doc link of auto-open feature. After setting your add-in auto-open, your add-in task pane can open automatically if the add-in is already installed on the user's device.