Help Phone goes into sleep mode while playing video

[Sovby]

When I am playing a video such as one from YouTube or any website my phone goes into what I guess is sleep mode so as to save battery life. How can I get it to play the whole video without shutting off?

 

[J_Hansen]

once video starts a quick push of the power button should lock the screen

 

[stretchttu]

Turn on Smart Stay in display settings?

 

[Rabi Dahal]

To fix it watch this video