const inventoryDetail = itemFulfillment.getCurrentSublistSubrecord({
     sublistId, fieldId: 'inventorydetail',
});

inventoryDetail.selectNewLine({
  sublistId: 'inventoryassignment' 
});

inventoryDetail.setCurrentSublistValue({
    sublistId: 'inventoryassignment',
    fieldId: 'quantity',
    value: value
});

//bin number's Internal ID
inventoryDetail.setCurrentSublistValue({
     sublistId: 'inventoryassignment',
     fieldId: 'binnumber',
     value: binNumber
});

inventoryDetail.commitLine({ 
  sublistId: 'inventoryassignment' 
});