You're just few steps away from downloading the ShowBox APK...
If it prompt "This type of file can harm your device. Do you want to keep ShowBoxv5.6.1.apk anyway?", click OK.
In case it shows "Install Blocked", click "Settings -> Slide Unknown Sources -> OK.
If prompt installation blocked this app contains code that attempts to bypass android's security protections. Click More details -> Install anyways) -> OK.