Can I open a bank account online in Thailand for my visa requirements?

You may be able to open an account this way with BKK bank in your home country....but not so here in Thailand as people have said already.

Go and see one of the visa places, I got mine for 4.000 baht

You have to show up in person. The basic requirement for a foreigner to open a bank account in Thailand is to have special relations with Thailand. This could be being married to a Thai national, working in Thailand (having a work permit), or "just" having a "permanent" stay in Thailand. "Permanent" stay will be proven by having a Residence Certificate from Immigration. At some/many Immigrations this is very easy to get even being a tourist staying at a hotel. The well-known exception from being easy is Bangkok Immigration. You can also pay an agent to help you but rarely necessary.

You must front up at a branch with lease (or certificate of residency), passport and a Thai person (preferably one who has an account at the same branch). Take the handout from immigration which states you must have xyz baht in a Thai bank account as your reason for opening an account.

No bank accounts in Thailand can be opened online.