There are many possible solutions for T117669: Invite users to enable Cross-wiki Notifications beta feature, at different levels of prominence. Some examples below:

Enable automatically
As the user uses cross-wiki notifications to move to a different wiki (where the beta feature is not enabled), the beta feature gets automatically enabled and a message is shown to communicate so.

Since the user got to that page for checking a notification, showing unrelated information can be distracting. The message is expected to be brief and just appear for a few seconds. In the mockup we show the message as a tooltip to connect it with the beta section (where the user can go to revert the change), but a bubble notification can be also used (mentioning the "beta feature" section more explicitly).

Ask to enable
Similarly, we can ask the user to enable instead of doing it automatically.

A user going through a list of notifications may get the message repeatedly, which can become annoying. The specific wiki name is used to avoid the confusion of the message to be repeated without apparent reason (which would make it even more annoying if the user thinks it is the same exact question).

Ask to enable from the panel
As the user uses cross-wiki notifications to move to a different wiki (where the beta feature is not enabled), we augment the notification panel to include an invitation to enable the feature.

This does not intrude on the main purpose of the user once the user lands in the page (i.e., read the information of the notification the user just clicked).
In addition, the invitation aligns with the user intent and information expectations: if the user was looking for more notifications (to continue processing them), we are providing an answer close to where the user may miss the information and an option to move forward.