If this is a matrix with multiple items being rated, I would have a side-by-side question type withe second column being the text box to provide reason for each rating. If it's a simple multiple choice question, add a text entry question below asking for the reason for the above rating.

You really have to make the text box a separate object that follows the Likert scale question. There's no elegant or respondent-friendly way to make the text field part of the same question.