From the latest addin preview documentation, it looks like delegate access of the addin should work in OWA.
We are using Office js to provide a custom outlook add-in for our customers. The user can use the outlook desktop client or the OWA to access the shared mailbox and install the custom add-in. But when using the addin in OWA with the preview requirement set, we are getting a permission error.
This is the error: "The operation on mailbox failed because it's out of the current user's write scope". (attachment error1.png).
Steps we did to try and install the addin in OWA
1. User logs into OWA with the primary user account (attachment point1.png).
2. User selects" open another mailbox" option and provides the shared mailbox information (attachment point2.png).
3. The shared mailbox opens correctly.
4. We select a message option (...) and go to Get Add-ins (attachment point4.png)
5. We select "My add-ins" in the left panel and scroll down to the "Custom add-ins" section and provide our custom addin url (attachment point5.png).



