LLC as S Corp treatment

I opened as LLC. I want it to be taxed as S Corp. Do I need to do I additional filing? And when is that form required 

 

I read it here https://turbotax.intuit.com/tax-tips/small-business-taxes/can-i-convert-my-llc-to-an-s-corp-when-fil...

 

that I can file 2553 at the time of filling  but I thought when I opened my LLC, I must file another form to let IRS know that I will be treating my LLC as s corp for tax purposes. So this is not correct?