The best way to do this is to use the upper levels in the Define new Multilevel List dialog
It is best to link that numbering to the default Heading # styles.
Using the above example where the Heading 7 style is used for the Appendix A, B, C, etc. in the Format Page Number dialog for the Sections of the Document containing the Appendices, in the Page Number Format dialog, you would check the box for Include chapter number and then select Heading 7 as the style that is used to start the chapter.
See https://shaunakelly.com/word/numbering/numberingappendixes.html