If I understand you correctlu, you want to invoke an unbound action, which is called "Action Import" since your action "ApproveBankAccount" does not bounded to any type.... According to the Odata Protocol, the folloding URL should be "POST": the "Account" should be included in Payload.
URL: http://example.org/service/BankAccountDomainService/ApproveBankAccount()
Payload: { abcde-123456 }
Aternatively, you can also design bounded action "Approve" on entity type "BankAccount", the posted URL should be:
http://example.org/service/BankAccountDomainService/BankAccount(abcde-12345)/NameSpace.Approve()