Roter is the word you are looking for, even if it is not slang and only colloquial. It is not the word you would use in good company, the posh word to use when you eat out with your boss or with the Queen instead of with pals is éructer.
Faire un rot can also be used although it is more of a phrase that will be used for babies. Bébé a fait son rot… (Baby has burped)
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