Rear Mount Drawer Slide Issue

I am building my first vanity for our master bath. Being that this is a temporary home for us but wanting to build my skill, I decided to try full extension drawer slides for the first time. I am going with a cheap option off amazon the corresponding rear mount brackets.

My issue is more in my lack of knowledge and inability to find any information online (shocking I know). My cavity is 18” but I have 16” drawer slides. I have heard the bracket allows several inches of flexibility but my brackets dont click or lock in to the drawer slides. I am wondering if i am using them wrong or if the bracket it meant to be loose and not have any sort of fastener from the slide to the bracket securing it.

The picture shows how far out i need to bring the slide in order to fit the cavity in the vanity. This same question would apply whether or not I was using an 18” slide in with the same bracket. It is just magnified with the extra space.

Thanks!