Normally, your livestream starts at the start time you selected in step 1 of the livestream creation. However, if you want to start the livestream manually, you can select this in step 4 of the livestream creation. To do so, check the "Start livestream manually" box.
After you have finished creating the livestream, you will be taken to the cockpit player, where you will find the red "GO LIVE" button in the upper left corner. With a click on this button, you can start the livestream.
However, this button only appears from the start time selected in step 1. That means you select the earliest time from which you would like to start the livestream manually.