I'm developing a Word Web Add-in with Office.js. This will work in conjunction with a existing VSTO add-in.
The VSTO installation process is as follows: A user clicks a link on a Web page. This opens an application on their machine which checks for the presence of the VSTO add-in. If it is not found, then the application prompts the user to install it.
There is a requirement to integrate the installation of the Office.js add-in with the existing deployment process if possible, so that the user does not need to install it separately.
The options for publishing and deploying Office Add-ins are listed here:
https://learn.microsoft.com/en-us/office/dev/add-ins/publish/publish
The add-in is an internal Word add-in. Therefore the applicable options for a production deployment are
The preferred option would be via integrated apps because it will be distributed to Mac users as well as Windows. However, since VSTO add-ins only work on Windows, it is possible we could support two distribution channels - the MS 365 one to facilitate Mac users and the SharePoint one to facilitate the integrated deployment.
Is this possible?
You should be able to do this with Open XML. You'll need to embed the add-in when building the document and set the taskpane to automatically open. The documentation for this procedure can be found in the article Automatically open a task pane with a document
There's also a full sample showing this for Excel workbooks. The principles should be the same.