Hi Mark,
If you want to use Reason Codes for Inventory Adjustments, you will need to use the Item Journal (as opposed to doing the adjustments from the Item Card or the Item List).
Using the search icon in D365 for Financials, navigate to "Item Journals". Right-Click on any of the column headers on this page like Posting Date or Entry Type, and from the drop down list that appears, click on Choose Columns.
In this list, you will find a column called "Reason Code", as shown in the screenshot below. Check this box to enable it and hit OK.


Now, you have the Reason Code on the page, and you can use it in your inventory adjustments. If you are not familiar with how to use the Item Journal, here is a simple set of steps.
Enter the Posting Date (date that the adjustment will be noted to the ledger).
Select the Entry Type (typically you will use either the Positive Adjustment or Negative Adjustment option, depending on the type of adjustment desired; I do not recommend using the Purchase or Sale option from here, unless you are priming the system during initial setup).
Enter a Document No. for the adjustment (an alphanumerical reference number of your choosing).
Enter the Item No., and the description auto-populates. Change it, if needed.
Enter the Location Code where the adjustment is being made.
In the Quantity field, enter the quantity that you are adjusting. You would always use a positive value, even if making a negative adjustment (for example, if you want make a negative adjustment of 2 PCS, enter 2 and not -2). The system would look at the Entry Type of "Negative Adjustment" and understand that the 2 is a negative number.
Select the Unit of Measure Code. The Unit Amount, Amount, Unit Cost, etc. will be automatically populated and do not need to be overwritten unless you have good reason to.
I would recommend using the Applies-To Entry if you use Negative Adjustment (it's a good practice, and here you can select which of your available item entries you are adjusting against. You could leave this blank and the system will still allow you to post this adjustment).
Finally, select a Reason Code from the reason codes you have already set up in D365 for Financials.
Repeat this process in the following lines if you want to adjust inventory for multiple products or a product in multiple locations at one time.
Finally, click the Post button in the ribbon, and you are done. If there is an issue, D365 for Financials will let you know.
I hope this helps. If my response has answered your question, please verify by clicking Yes next to "Did this answer your question?"