A POST is preferable for login request, because the authentication information will be sent in the HTTP messages body rather than the URL. Although it will still be sent plain text, unless you’re encrypting via HTTPS.
GET method data is sent to the server followed by the URL which will be seen to everyone.
Both GET and POST method are used to transfer data from client to server in HTTP protocol but main difference between POST and GET method is that GET carries request parameter appended in URL string, while POST carries request parameter in message body which makes it more secure way of transferring data from client to server in HTTP protocol.