This dialog can be opened by right-clicking on the title bar of an embedded Excel workbook and selecting the Properties... action from its context menu.
The workbook can be saved as an external Excel file linked to the project or as an internal object in the QtiPlot project. If you choose to save it as an external link it is beneficial to save the Excel file in the same folder as the QtiPlot project or in a subfolder under it and then set Excel file path relative to QtiPlot project path to make them more portable.