How to indicate a single required field in a big form?

Foreword: this form is a UX nightmare in itself. My task is not to make it better, just to show a required field.

I have a form in which the user can add multiple lines to a dynamically added group of data called "activities". These lines ultimately become the budget for said activity.

Each of these lines come with a field called "justification". This field is required to be entered by the user, or else he won't be able to save his work.

Here is what it looks like:

The highlighted column is the required field. As previously stated, this is the only field in the whole form that will prevent the user from saving.

So, how do I indicate to my user that this field is required in a sea of other fields that are not?