There are 3 ways to start a Datasource request, and you can even use a combination of them.
For example, you can have a List of Users Datasource to Auto start when screen First Loads but also have an On Click Event Action on a button that starts the same Datasource again in order to refresh the Users.
You can enable the Auto start when Screen First Loads option so that the Datasource will start automatically during the Screen loading.
Tip: Use this option when you need to fetch the data that needed to be displayed, and not for cases like Register user where you should have a button that starts the Datasource request.
If starting a Datasource request requires more complex logic (for example, it is dependent on a condition), you could implement it using Code blocks.