Hello, you have that option before calling a microflow in a button for example : 

Sometimes this will not be enough because you will want to trigger a microflow if the user click on yes or no, also sometimes that option is not available, in that case you will have to create a popup with two buttons yes and no, you will then display that popup in your microflow with a show page action. 

You can use a Nanoflow for this using the ShowConfirmation action from the nanoflow commons module.