Dantes You basically want site:reddit.com search when using reddit but it is not as simple.
'reddit json api'
'teddit vs reddit'
there is a whole range of queries where you do not want site:reddit.com behavior.
The best way to be explicit with Kagi is to create a Reddit Lens. Next best is simply boosting reddit results. Even for your query there is a number of reddit results grouped together, just not at a first position. Would this work?