public static final String AUTH_URL = 'https://test.salesforce.com/services/oauth2/token';
public static final String CONSUMER_KEY = '3MVG97wqanbUM37IrVAYa3ZeAtHO0PIQOmIJOQPSG3C7ZlKqV6qQ28LOLPkefh_JEfzMwrWX2JVJ.k0o6TgEr';
public static final String CONSUMER_SECRET = '5259521906564034481';
public static final String API_USERNAME = 'admin@unisource.com.salesiq';
public static final String API_PASSWORD = 'P@ssw0rd1';


String reqString = String.format('grant_type=password&client_id={0}&client_secret={1}&username={2}&password={3}',
                                 new String[] {EncodingUtil.urlEncode(CONSUMER_KEY, 'UTF-16'),
                                     EncodingUtil.urlEncode(CONSUMER_SECRET, 'UTF-16'),
                                     EncodingUtil.urlEncode(API_USERNAME, 'UTF-16'),
                                     EncodingUtil.urlEncode(API_PASSWORD, 'UTF-16')});


System.debug(reqString);
HttpRequest authRequest = new HttpRequest();
authRequest.setMethod('POST');
authRequest.setHeader('Content-Type', 'application/x-www-form-urlencoded');
authRequest.setBody(reqString);
authRequest.setEndpoint(AUTH_URL);

Http http = new Http();
HttpResponse authResponse = http.send(authRequest);
System.debug(authResponse.getBody());