Can I enter Thailand with a one-way ticket and get a 30-day extension?

Whoops, my apologies

I did just check the eVisa system saying I'm 'murican but IN Portugal and it will let me apply for a tourist visa that way

SO you could go to the thai consulate website in Lisbon, read the requirements for the tourist visa, show proof you're going to thailand FROM Portugal and you MIGHT get a tourist visa approved

So to recap,

Nope, can't use eVisa system <- saying you're in the US

check with your air carrier about flying in on a one way ticket visa exempt entry

you can get one 30 day extension on your entry for 1900baht after you're here

Just curious what the travel history had to do with your question?

Be that as it may, 😉

Who ever told you about the eVisa was correct, you can only use the eVisa system IN your country and most consulates state that on the website.

AND

they're also right you'd show air travel from your home country TO thailand to use the eVisa system even if you spoofed it and said you were in the US

SO

What you're intending to do is fly in without any visa and get stamped in for 30 days free on what's called a Visa Exempt entry.

AND

you're almost right, you can indeed get a 30 day extension on that free 30 day entry

BUT alas

it's not free 😕 it's 1900baht at the immigration office in thailand that serves the area where you stay.

Now the problem you might have is SOME airlines have a policy that if you're flying to thailand without a valid visa (entering free stamp visa exempt) you need to show proof of onward travel within the number of days you'll be stamped in for (in your case 30 days) or you can't check in for your flight.

CHECK with your air carrier and if they have no issue with you flying in without onward travel you'll be fine once you land here