RenderTarget in scripts

I know how to use the editor to create a render target, but it always gives me the wrong aspect ratio, I want it to be the exact ratio as the screen. so I know that I need to make a rendertarget through code, just like a full screen effect. but the thing is, that I'm not sure how to do it exactly? I've looked around, and i've seen like the "Rendertarget.create()" function and the constructor, but I keep getting errors, and all in all I'm not sure how to do it, any help is much appreciated!