My dog just ate dark chocolate

Okay so it's 3am in the morning and my dog is already having an early symptoms like heavy breathing, fast heart pulse, diarrhea, seizures and I can't sleep because I'm scared. First thing in the morning is im gonna buy hydrogen peroxide 3% pero I feel like thats already too late cause he ate a bunch of chocolate. Yung signature cake sa goldilocks, yung buong top niya and one big bite yung nakuha nya, he's a shih tzu and 6kg ata siya as far as I remembered. Idk what to do I researched a lot of what to do and dont do, im just 16yrs old and im freaking out.

Ps: The dog was smart and not neglected, he can do ninja moves. We have 2 ref in our house one on the kitchen and the other one is nasa storage room both are functioning tapos the guest or gisitors in our house forgot to close the door sa storage room and sakto may signature cake sa ref and nabuksan siya ng aso ko and he ate pero napigilan naman siya but idk what to do anymore. Pelase help and please be nice.