First you need to setup a User Defined Field (Payer>Payer Edit>User Defined Fields) called XSICK.  This field is setup as an template.  To access the example, double-click on the first blank line on the User Defined Fields screen.  Click Templates, highlight Third Party Sick Pay, click Select, click Apply and then click OK to add the settings for this field.  When you add the employees check, enter the sick pay amount in this field.