In Kubuntu 18.04, it is very simple. Open "System Settings" in the main menu (click on Kubuntu icon on the bottom left to find this option in the "Favorites" section) then click on "Shortcuts" on the left menu of the System Settings window.
Inside the "Shortcuts" main window, select "Custom Shortcuts" and then click on the "Edit" button at the bottom of the middle left tab section. Select "New/Global Shortcut/CommandURL". By doing this, you will create a new custom command for the keyboard that will show under all the other commands that are already created. You can rename the new command by double-clicking on the new command name. Good practice keeping things tidy but it is up to you.
Now the key binding part. Select the newly created command and on the main window, click on the "Action" tab. There will be only one Command/URL field. In that field you simply type "dolphin" (without quotes). Next you click on the "Trigger" tab located besides the "Action" tab on the top of the window and then click on the button to the right of the word "Shortcut:", the button will change its name to "Input". Now it is the time to set the combination that you want on your keyboard. In this case, just hit Meta+E (like in windows to open win explorer, yack..) on the keyboard to bind the key combo to the new command and that's it.
Now you have your personal shortcut to open dolphin through the keyboard.