JavaScript POST non-API request with repeated keys in payload

Javascript. POST request. It is not an API request (axios or fetch), but the old classical request.

I need a payload like ...&_selected_action=5&_selected_action=10.
So I think I cannot use a form submitting technique here.
With a single occurence ...&_selected_action=5 I am able to do it, everything works well.
For the repeated value I can create the string by hand or by URLSearchParams().toString() but I am not able to send it.

Any idea?

More details. I need create this request in Django+Vue application which partially replaces the old school Django Admin. The ListView is replaced using Vue and now I want to reuse the actions where Django Admin uses Post request formatted as above. So with single selected row in list I am able to do it, but with 2+ selected rows I cannot find a good way how to do it.