Can I extend my 90-day stay in Thailand online or do I need to go in person?

You don't extend anything. You report you are inside Thailand for 90 consecutive days to your local immigration office. Normally you have to do the first 90 day report in person but you're out nothing by trying to do it online, just make sure you have time to do it in person if that doesn't work

I think you're talking about 90 day reporting. First time has to be done in person, after that you can do it online.