Asp.net Web Api Authentication and password encryption - asp.net

Question

I have a autentication with the web api working, but is using the email and password to autenticate, how can i change so that web api uses a username field and password for the autentication?
Another thing, how can i change the encrytion method for the password?
The Autentication is made this away:
public async Task<string> LoginAsync(string username, string password)
{
try
{
var keyValues = new List<KeyValuePair<string, string>>
{
new KeyValuePair<string, string>("username", username),
new KeyValuePair<string, string>("password", password),
new KeyValuePair<string, string>("grant_type", "password")
};
Settings.Serverdown = false;
var request = new HttpRequestMessage(HttpMethod.Post, "http://192.168.44.22:56479/Token");
request.Content = new FormUrlEncodedContent(keyValues);
var client = new HttpClient();
client.Timeout = TimeSpan.FromMilliseconds(6000);
var response = await client.SendAsync(request);
var jwt = await response.Content.ReadAsStringAsync();
Settings.LoginSucess = response.IsSuccessStatusCode;
Settings.Username = username;
JObject jwtDynamic = JsonConvert.DeserializeObject<dynamic>(jwt);
var accessToken = jwtDynamic.Value<string>("access_token");
Settings.AccessToken = accessToken;
}
catch (Exception e)
{
XFToast.LongMessage("Cant reach the server");
Settings.Serverdown = true;
}
var accessToken2=Settings.AccessToken;
return accessToken2;
}
Thanks