Hi,
To have it automatically done by the application you should have an inventory transaction , that will have an inventory dimension same with your sales order line, with a Status Receipt "On Order" for the same item.
That means, when you want to reserve items you should have item A in a transfer order or purchase order, or inventory transfer journal etc. that will arrive to the same site/warehouse which is on your sales line.
In that case, the remaining 20 will have a status issue of "Reserved Ordered" instead of "On Order"
To do this AX will create inventory marking by itself. When you accept the items, Reserved Ordered status will be updated to "Reserved Physical".
One last thing, the checkbox labeled "Reserve ordered items" in Inventory Management/Setup/Inventory and warehouse management parameters controls if "Reserved ordered" status will be used or not. If this checkbox is not ticked then, application won't bother reserving items against incoming inventory. In such a case, if quantity you would like to reserve is higher than what you have available in related inventory dimension, application will throw error and stop the transaction.
If you need more information about "Status Receipt" and "Status Issue" values of inventory transactions, you can check Tim's blog post here: The Inventory Transaction Status in AX 2012.