How to add the multi-select button to a custom view

[RethaLam]

I have a custom view and I would like to use the multi-select button that one sees on the FulfillmentLinesView form where if in singleSelect mode and it is clicked, then the dataList changes to multi-select mode and if it is in multiSelect mode, then it changes the dataList to singleSelect.

If I look at "RetailSDK\POS\Web\Views\CustomerOrder\FulfillmentLineView\FulfillmentLineView.html" I cannot really see code that was added to accomplish the switch between the 2 selection modes.
Now on the FulfillmentLinesView when in single selection mode, it doesn't show the checkbox.
However for any custom view, there is always a checkbox,regardless if the datalist is in single or multi-select mode.
So I'm not after the appearing and disappearing of the checkbox.
I just want to also add the Selection button to the AppBar, and then flip between the 2 modes to be able to keep the custom view to behave the same as the standard view.

Any help is much appreciated.