You can do it 2 ways with 2 different types of translucency. One is native KDE, but it’s not true translucency. The other is true, but requires a simple applet you download from the the ADD WIDGET panel. 
But this it off topic. Take a look in ADD WIDGET/DOWNLOAD NEW APPLETS or in [Latte Dock] The end of an era (designing a Latte-Dock free alternative dr460nized setup)