In the projectile selection menu, you can select which custom projectile your wand should shoot. If you have a single custom projectile called 'blaze_ball', it should look like this:

This menu has a 'Cancel' button and a button for every custom projectile that has been created already. So if you haven't created any custom projectiles yet, you will only see the 'Cancel' button.

Clicking on the 'Cancel' button will take you back to the previous wand edit menu without selecting a new projectile. Clicking on any of the projectile buttons will select that projectile and then take you back to the wand edit menu.