How to modify the following shiny code to obtain row indexes selected by checkbox?

I am interested to have a data table as following in my shiny App. After I select the rows using the check boxes, I would like to continue working on the selected data. The problem now is I don't know how to get the index of the selected rows from this example. Can you please help? Thank you very much!

How do I make persistent checkboxes with DT::datatable in Shiny?

I think it has to do with the callback function. But I couldn't figure it out.