Yacy only crawl / index pages which contain a certain keyword


I am trying to index a site A which shares it’s domain with many other sites B-Z.

https://example.com/blog/3765 - Blog article of site A
https://example.com/blog/4836 - Blog article of site B… C etc

I am trying to seperate these pages from one another, since I only want to index site A and none of sites B-Z.
Site A contains (as its template) a very specific string so it seems natural to me to just do a fulltext seach for this specific string:

if(page.html.contains(“header string of site A”)) {

How can I achieve something like this with yacy?