What is the issue you are encountering?
From the manual:
Some data in JSON format that lists the widths of individual characters. This allows improved text layout by using narrower spaces for narrower characters. The spacing data also affects the display of the text in the Layout View. The data is an array of pairs. Each pair is a width, and then a string of all the characters that width applies to. For example the pair [10, "aeou"] will set the width of the characters a, e, o and u to 10 pixels. The characters are case-sensitive, allowing you to choose different widths for uppercase characters. You can also set the width of the space character. Each pair must be listed in an array, e.g.
[10, "aeou"], [12, "mvw"]].