Have you tried searching this in Google?
There are few free addins and external tools available in the internet.
1. Here is a detailed blog on
ApexSQL Complete[
^]
2.
dbForge SQL Complete[
^]
Hopefully, it helps.
Note: I don't recommend to use third party plugins or tools because sometimes they come with much more headache than advantage. You may realize that upon uninstalling. Best option according to me is to upgrade your SQL Server to higher version which have inbuilt autocomplete facility.