I believe that this is the third-party add on (not stock UBB.threads code) which you're using -
https://www.ubbdev.com/forums/ubbthreads.php/topics/318555/1.html

How you would add attribution, would be to write within the "Format of the posts:" box, something similar to the following -

Code
<b>%%TITLE%%</b>
%%DESCRIPTION%%
SOURCE: <a href="%%LINK%%">racer.com</a>

Current developer of UBB.threads PHP Forum Software
Current Release: UBBT 7.7.5 // Preview: UBBT 8.0.0
isaac @ id242.com // my forum @ CelicaHobby.com