I just started doing Unreal engine 4, be easy on me please. I’m not currently working on a precise project, I just want to replicate stuff that I know from other games I played, like this feature. I want that when I press the ALT key my camera unlocks and the mouse cursor to show up and when I press it again my camera locks back and the mouse cursor is hidden. I created the events for when I press the keys I just don’t know what to use to make the cursor show up and off. I’m currently on the third person template. UE4 community looks awesome and I can’t wait to join it, thanks in advance! 
I think you have to use Set Input Mode Game and UI or Set Input Mode Game only
Welcome to da club 
By unlocking the camera, I assume you want the camera to stay in its current place, so you can use the mouse cursor to do stuff? I would create a bool, and check on the LookUp and Turn events if this bool is true. If not, add the input like youre doing now, if it is, do nothing. The bool should be toggled when pressing the ALT button. Additionally, there is a node called GetController, if I remember correctly you can drag off of it and call ShowMouseCursor (its a boolean, you may have to cast to your player controller first, i dont have access to UE while writing this). This toggles showing the mouse cursor on screen.