Batch: Pipe loop's output into variable, than check with if else

I have the following batch script:

@ECHO OFF

SET "TLD=.com"

FOR /F "tokens=*" %%i in (%cd%filesdomains-win.txt) do (
  whoiscl -r -n %%i%TLD% | FINDSTR /I /C:"Registrant Name">NUL 2>&1
  SET "OUTPUT=%%i%TLD%"

  IF not "%OUTPUT%" == "" (
    ECHO Registered: %%i%TLD%
  ) ELSE (
    ECHO Available: %%i%TLD%
  )

)

I want to check if FINDSTR's output is matching Registrant Name, than ECHO Registered in an if else. How can it be achieved?