In your firestick or android box Clcik search
Type in Mx Player
You will see MX Player.
Download the MX Player and install it. Then close the window no need to open it.
Open the slacker App
Open the menu panel
Go to settings.
Then playback
Scroll down to choose external player for individual channels
Pick the MX player from the drop down
Pick your channels from the list that pops up