Hi Sourabh ,

List Append, adds a element at the end of the list. You can add your element in your list simply by using ListAppend action in your flow. There many system defined list actions are available that help you to work with list items.  

Please check helping links. 

1.  https://success.outsystems.com/Documentation/11/Reference/OutSystems_APIs/System_Actions#ListAppend

2. https://www.youtube.com/watch?v=LoUG74HtRng

Cheers,

Rahul