You can select inboxes from the Permissions sections.
Click your profile avatar from the lower left-hand pane > select My Profile > navigate to Permissions in the upper left-hand pane. From here, you will see all of the inboxes to which you are assigned.
You will see a green check mark symbol on inboxes you have selected. To add another inbox, hover over the inbox and click the lower left transparent selector checkbox - notice Billy’s inbox below.