How to set multiple values in a custom NetSuite mult-select field

Question

I am trying to set multiple values using SailPoint in a NetSuite multi-select field. Currently the script below allows me to set one value in the field. If I try to set a 2nd value in the field, it overwrites the first value instead of adding the 2nd value. How do I need to update the code below to assign a second value in the field instead of overwriting the current value?

{
"custentity_employee_property_assign_v2": { 
    "items":[{ 
    "id" : "$plan.empPropertyid$" 
    }]
}
}