Thank you for your helpful suggestion, Florian. "https://boomkat.com/products?q[keywords]=Pop" works in the browser and I'm no longer receiving the uri error. However, I'm now receiving a 12029 "a connection with the server cannot be established" error. Progress, I think...
[Name]=Boomkat
[BasedOn]=https://www.boomkat.com
[IndexUrl]=https://boomkat.com/products?q%5Bkeywords%5D=%s
[AlbumUrl]=
[WordSeperator]=%20
[IndexFormat]=%_url%|%Artist%|%Album%|%Catalog#%|%Label%|%Genre%
[SearchBy]=$regexp($regexp(%album%,'\s+\[.+\]$',,1),'[+!(){}\[\]^"~*?,:\\\\]','%20')
[Encoding]=url-utf-8
Best regards to you.