Downloading 3rd party screen savers

I have a fun screen saver site that is safe, but it’s third party and not on the Apple app list. I can install it but it won’t open and work. It’s just a blank screen. But if I try the “preview”, the screen saver works. Of course this only works with the preview. If I try to start the screen saver again a blank screen. Does anyone know how to get past the Mojave security settings? I have tried the instructions for by passing Gatekeeper to temporarily allow apps from anywhere but that does not work either. Can anyone tell me how I can download third party apps and have them work? Thank you, I am new to Mac. I have always had windows PC.