The Board's backlog displays tasks that someone has added to the scope of the Box. There are different ways in which you can add tasks using the Board module.
You can't add tasks to Boxes in the "Closed" status.
Create new tasks in the host platform
You can add new tasks in the host platform (the host platform is the Jira instance you are currently using) in a few ways.
Click the "+Add" button and choose "Create task".
Click the "+" icon to add a task.
Once the task is added, it is assigned to a Box and will appear in the Backlog when the "Show already planned task" feature is enabled:
Infobar > Backlog
Expand the Infobar section and navigate to "Backlog". To add a task, click the "+" icon.
To create a basic task, click the "+Add" button and choose "Create task > Basic task".