If you right-click on a in-text citation, you normally have an option 'Edit Citation' (not 'Edit Source'). In the dialog you have the option to add a page number. The Harvard styles on this site will automatically pick them up.
If you want to do it in your own style, you will need to process 'CitationPages' rather than 'Pages'.
If you want to do it in your own style, you will need to process 'CitationPages' rather than 'Pages'.