One of the ways to attach a sling plate to your shotgun is to get a sling plate which is installed between extentsion and magazine tube. There are many sling plates available, here is one from S&J Hardware:
This sling plate is good, it enables you to attach sling if you want one.
More information on S&J Hardware website.
Remington 870 3 position front sling plate
Here is another sling plate from S&J Hardware, it is similar to previous one but it enables you to choose one of three positions for your sling:
More information on S&J Hardware Website.
I always recommend S&J Hardware products because they are well made.