There is a limitation with the confirm microflow widget where it can't get the selection of a datagrid as input parameter.
Your best bet is to create a pop-up form with a dataview of the selected object and place a confirm delete and cancel microflow/button there.
I have done it at the cost of an extra boolean. Make a custom select button that sets the boolean and then open a custom form. In that form delete all the records with the boolean set to true after confirm or remove the boolean after a cancel. Not the ideal sollution, but it works.
It is a pity that the confirm microflow widget does not support datagrid selections. It would make life a lot simpeler.
Alternative is to add an listening dataview, and but (just) the button in there. Layouting it in a nice way might be a bit tricky though.