Set sub list value in the suitelet form.

Set a single value in the drop down list

var form = serverWidget.createForm({
    title : 'Simple Form'
});
var selectField = form.addField({
    id : 'custpage_selectfield',
    type : serverWidget.FieldType.SELECT,
    label : 'Select'
});

selectField.addSelectOption({
    value : '',
    text : ''
});

selectField.addSelectOption({
    value : 'a',
    text : 'Albert'
});

Set an object in the drop down list

                    var itemInternalId = sublist.getSublistValue({
                        id: 'custpageprojdept',
                        line: k   
                    });  // Read the item id 
                    log.debug("itemInternalId", itemInternalId);
                    let vendorDetail = vendorDetails(itemInternalId);  // call a function to get the preferred vendor details.Return of this function will be objcet
                    log.debug("vendorDetail", vendorDetail);

                    SetVendor.addSelectOption(' ', ' ');

                    for (var j = 0; j < vendorDetail.length; j++) {
                        SetVendor.addSelectOption(vendorDetail[j].id, vendorDetail[j].name); // Set object in the form of key value pair
                    }

                    log.debug("preferedVendorDetails",JSON.stringify(vendorDetail))