I have the same issue but I couldn't do what you mention, is there any related manual?
See FR's manual how to create Master-Detail report: https://www.fast-report.com/public_download/docs/FRVCL/online/en/UserManual/en-US/Creating_reports/Report_with_two_data_levels_(master-detail).html
Rich Text Editor.
To edit a paragraph's style, hit tab to get to the paragraph menu. From there you will be able to pick one style. Nothing defaults to paragraph.
An inline formatting menu will show up when you select text. Hit tab to get into that menu.
Some elements, such as rich link embeds, images, loading indicators, and error messages may get inserted into the editor. You may navigate to these using the arrow keys inside of the editor and delete them with the delete or backspace key.