🚀 go-pugleaf

Click the text in question to place the cursor in the tag. 

Press Ctrl+M. 

Go to the numbering tab. 

Voila. 

BTW, if you want an appendix tag, also, you'll have to create one. 

With "ChapterTitle" active in the Paragraph Designer, click Commands > New Format. Type "AppendixTitle" for the Tag name; click Create. In the Numbering tag, change "H:Chapter" to "I:Appendix." 

Thereafter, apply the AppendixTitle tag wherever you need it. 

Note, fwiw, I'd name your titles TitleChapter and TitleAppendix, so they appear next to each other in the tags. 

Note, Ctrl+M and Ctrl+D get you paragraph and character designers for paragraph and character tags. In addition, cross-references and tables have philosophically similar setups. You should get comfortable with these things because they are important to the strengths of FrameMaker. 

I also strongly recommend: 1) Adobe FrameMaker Classroom in a Book and 2) some formal training. But, I hope I've given you a leg up. 

Cheers, 

Sean