Only user have permission Edit can inputted the values.
In the Issue Screen, Jira have two mode ( Issue Create, Issue View).
In Issue Create mode, permission Edit is required. The user can input the values in the custom field.
In Issue View mode, the user have permission view or edit will see the value of the custom field.
Click on this custom field to go Edit mode.
if user don’t have permission:
Multiple Line Field Security get the value from the inputted custom field on Issue. The application will process the value (encode to store it, decode to show it).