The user guide for Dynamics 365 App for Outlook explains how users can load the app for first time. How one can Track and Set regarding to link with D365 CE records. A quick tour of UI can help familiarize users with what they see in the version 9.0.

A common question arises among customers transitioning from Dynamics 365 for Outlook (Com Addin). How to track Tasks? There is no Dynamics 365 button when I navigate to Tasks or open a Task. While Track or Set Regarding is not directly available on Tasks, there are few options offering same functionality.

Category Tracking

Category tracking was introduced for D365 Online version 8.2 and later. Category Tracking relies on Server-side sync and can be used with or without App for Outlook. However, using Category Tracking with Dynamics 365 for Outlook (Com Addin) is not supported and not recommended to enable it for D365 orgs those use Dynamics 365 for Outlook (Com Addin). Category Tracking doesn’t offer Set Regarding, but user can Track Tasks assigning Category then open the Task record in D365 to populate regarding value. Enabling category also helps as visual cue to differentiate tracked items.
Category Tracking requires enabling via OrgDBOrgSetting. Here is the document describing, how to enable it. Update: Category Tracking is enabled by default for version or higher.

Create D365CE Tasks from App for Outlook

App for Outlook allows creating D365 activity records. There could be variation in steps depending on how the App for Outlook is customized in an environment. At high level, here are couple of options for OOB experience.

Quick Create

Use the Quick Create -> Activities -> Task.

            Timeline wall

                Create (+) button on Timeline.
This approach creates Tasks directly in D365 and will synchronize with Outlook by the virtue of Synchronization filters and SSS sync cycle. Users can choose D365 record as regarding value. In fact, creating from timeline populates the regarding field like D365 web app does it while creating related records.
Hope this helps!