AutoOpen

An office add-in can automatically insert a task pane into a specific file.
Whenever the user (or a different user) opens that particular file, the task pane can be displayed automatically.

(this applies to Excel, Word and PowerPoint)

link - learn.microsoft.com/en-us/office/dev/add-ins/develop/automatically-open-a-task-pane-with-a-document 
link - stackoverflow.com/questions/48305027/auto-open-taskpane-opens-sourcelocation-url-instead-of-taskpane-with-office-au 

If the add-in is installed, then the task pane will open automatically.
If the add-in has been removed, then the New Office Add-in task pane will be displayed.
SS

<Action xsi:type="ShowTaskpane"> 
    <TaskpaneId>Office.AutoShowTaskpaneWithDocument</TaskpaneId> 
    <SourceLocation resid="Contoso.Taskpane.Url" /> 
</Action> 

Tag the document on the client side

Office.context.document.settings.set("Office.AutoShowTaskpaneWithDocument", true); 
Office.context.document.settings.saveAsync();