Travelling from Colombo-Beijing-Singapore

I'm spanish citizen and I will be only 24 hours in Beijing (I already made booking in Hilton hotel), my question is I try to notify in advance to my airline company ticket (in this case Sri lankan Airlines) but they said they can't inform inmigration as recommended below

You can put forward your request for transit to your airline in advance, so that the airline can declare this to the Immigration Inspection prior to your visit. You will be granted a transit permit at the airport if you meet all requirements.

will be that a problem? where I should informed the company? when I check in on the flight? or I can apply for the 72hours avisa even if I don't notify the airline company?

thanks