Access to the return-to-work form is managed within each sickness reason.
To set this up:
Click Access Button
then click Employees.
Select the relevant employee then click Planner.
Click add a sickness then within the Reason dropdown, select +(Manage this list).
Select a sickness reason, then tick Return To Work Form Required option.
Click Save.
From now on, when an employee has added the sickness with the reason you made this set up for, they receive a daily reminder from the first day they come back to work until they fill in the return to work form.