I found it.
I just overwrote parameter on Report Server, in the parameters section
But then I realized it's not good enough.
What if the user searches for a keyword, runs the report and then wants to go back to <ALL DATA> mode?...
He would need to type <SELECT ALL> again ? ....hmmmm
Here is what worked for me.
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On Report Server I set the parameter to:
Has Default : check
Default value : blank
Null : check
Prompt User: check
In the stored procedure I default input parameter to NULL.
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I still don't like NULL small check box at the front
but this is the best I can do now.