You could make an HTTP POST request to https://yourdomain.tld/login
with the following JSON body:
{
"identification": "username",
"password": "userspassword",
"remember": false
}
The problem however is that this call requires the CSFR header to be part of the request. If your missing the CSRF token header you'll get an error. It's for sure something you could play with though.
Other than that HTTP request though to my knowlege there is no API request method for dealing with user logins and there is for sure no OAuth2 server built-in or anything of that nature.
One of the other developers may know more about the API and it's methods than I do though.