OK this is something I do know a bit about - for anyone else reading and @Peoniesandcats .
The best site to test with is Ancestry. This is for two reasons - first, they have the biggest database and so you are more likely to get matches. Secondly, they allow you to extract your raw DNA file (which is just a list of numbers) and then upload it to other sites like MyHeritage, FamilyTreeDNA, GedMatch etc. Ancestry do not allow to test with other sites and then upload to them. 23andMe has traditionally been more popular for health testing only - do you have a gene which makes you more likely to develop X condition. It has a stronger north American database but if you are looking for relatives in SE Asia might not be that useful.
Few other points which I have learned the hard way.... You might not have any success at all. Some communities are much keener on DNA testing than others. Australia, North America - lots of people who know that originally their relatives came from somewhere else embraced DNA testing. British people aren't so keen, especially those with a white British background whose grandparents were all British born and there is no suspicion that further back other people came from overseas. So I tested a few years ago and got hardly any matches despite my granddad on the paternal side being on of 11 and his father being one of 15(!!). I have hundreds of second cousins but only far more distant matches. So you might not get any answers.
Also when you DO get matches, you then have to start building family trees to work out how everyone fits together. This is really hard when you have two unknown parents, easier when you have an unknown father and can just hone in on the matches which you don't recognise as being on your mother's side. Building trees also means looking at records of birth/death/marriage, census and other official records and these are not so widely available for SE Asia as they are for Europe and N America.
I think you need specialist help here, a genealogist who specialises in SE Asia, or a Facebook group for people adopted from there. There is also a site called MyPreciseDNA which appears to be based in Malaysia, but I don't know it to comment one way or another.