If an employer informs you that they wish to opt in to (or, if they are an "entitled worker", to join) a qualifying pension scheme, you must record this.
To record an employee opt in:
Go to Employee Notifications (or the Employee Notifications tab on the employee record).
Add a WEOI (opt in/join pension scheme) notification.
Coins ERP+ takes this into account during the pay calculation, and adds the pension scheme record based on the WEOI.