Sidenote: I’m using the Angular integration of the Microsoft Analysis Services version of the Flexmonster tool.
Thank you for writing to us.
The submenu can be opened programmatically by dispatching an appropriate event. Here is the jsfiddle example: https://jsfiddle.net/flexmonster/sgpb6cdu/.
Please note, that there is no need to open the context menu for pre-setting “% of Row” aggregation.
You can set the appropriate aggregation in the slice object and then apply it by using our setReport() API call. Please find more information here: https://www.flexmonster.com/api/slice-object/, https://www.flexmonster.com/api/setreport/.
Let us know in case of any other question.