i'm doing bin assignments on an item receipt like this (myReceipt is standard mode):

var binDetail = myReceipt.getSublistSubrecord({
			sublistId : 'item',
			fieldId : 'inventorydetail',
			line: e
		});
					
binDetail.setSublistValue({
	sublistId : 'inventoryassignment',
	fieldId : 'binnumber',
	value : parseInt(items[itemLine][1]),
	line: 0
});

binDetail.setSublistValue({
	sublistId : 'inventoryassignment',
	fieldId : 'quantity',
	value : 1,
	line: 0