Add a free text justification box for some expense types

Summary:

Can anyone advise how to add a free text justification box when entering only some expense types.

Content (please ensure you mask any confidential information):

We are looking to set up a justification box to capture why this spend is going through expenses and not another route.

Version (include the version you are using, if applicable):

26A

Code Snippet (add any code snippets that support your topic, if applicable):