I did it though (sort of):
Dolphin → Configure → Configure Dolphin → Startup → “Show on startup” I put in the field just recoll:
So, whenever I start Dolphin, I just click the address bar and write my search phrase. 
This is a bit more convenient.
What do you think?
UPDATE:
I even added “recoll:” to places… so if I want to search I click recoll from the left and search.
You are amazing @dalto.
You made a lot of effort and was so patient with me. I highly apreciate all what you do/did.
