Instead of clicking on the "Apply" button on the adjustment line, use the APPLY button next to the CREATE button.

1. Select the line (or lines) where the counted quantity should be applied.

2. The APPLY button appears. When clicked, a new pop-up window is shown with an "Inventory Reference/Reason" field.  It already contains the values "Inventory Adjustment - Date of Adjustment". Add the adjustment reference next to the date and apply the adjusment.

3. Click on the History link on the adjustment line. Here, you can see the reference used: