I'm gonna tell you the steps to achieve that.
First of all to create a microflow. On the explorer, right click> add microflow
After add if you want to add a element to a list you should receive that element and the list as paremeter. For sure, the list can be obtained from a retrieve but I'm gonna show an example with two paremeters.
You should define the parameters as list and object:
There you select the entity of the paremeter and later on you will be able to add the element to the list like this:
Using the change list activity from the microflow toolbox you are able to add a new element.
Hope this helps you.