var result = {};
var application = "";
var vendor = "";
var businessunit = "";
var user = "";
var pass = "";
var authCode = window.btoa(application + "@" + vendor + ":" + businessunit);
$.ajax({
    "url": 'https://',
    "type": 'post',
    "contentType": 'application/json',
    "dataType": 'json',
    "headers": {
        'Authorization': 'basic ' + authCode
    },
    "data": JSON.stringify({
    "grant_type": 'password',
    "username": user,
    "password" : pass,
    "scope": 'AdminApi AgentApi AuthenticationApi PatronApi RealTimeApi'
    }),
    "success": function (resp) {
        result.access_token = resp.access_token;
        result.token_type = resp.token_type;
        result.resource_server_base_uri = resp.resource_server_base_uri;
        result.expires_in = resp.expires_in;
        result.refresh_token = resp.refresh_token;
        result.scope = resp.scope;
        result.refresh_token_server_uri = resp.refresh_token_server_uri;
    },
    "error": function (XMLHttpRequest, textStatus, errorThrown) {
        alert("Failed to retrieve token.n" + XMLHttpRequest.status + ' '
            + XMLHttpRequest.statusText);
    }
});

public class AuthCallout {
    public void basicAccessTokenGeneration() {
     HttpRequest req = new HttpRequest();
     req.setEndpoint('https://');
     req.setMethod('POST');

     // Specify the required username and password to access the endpoint
     // As well as the header and header information

     String username = '';
     String password = '';
     String vendor   = '';
     String businessunit ='';
     String application ='';

     Blob headerValue = Blob.valueOf(application + '@' + vendor + ':' + businessunit);
     String authorizationHeader = 'BASIC ' +
     EncodingUtil.base64Encode(headerValue);
     req.setHeader('Authorization', authorizationHeader);


    JSONGenerator gen = JSON.createGenerator(true);

    gen.writeStartObject();
    gen.writeStringField('grant_type ', 'password');
    gen.writeStringField('username', username);
    gen.writeStringField('password',password);
    gen.writeStringField('scope','');
    gen.writeEndObject();
    String jsonS = gen.getAsString();
    system.debug('DAta in '+str);
    system.debug('Data in gen'+jsonS);
    req.setBody(jsonS);
     // Create a new http object to send the request object
     // A response object is generated as a result of the request

     Http http = new Http();
     HTTPResponse res = http.send(req);
     System.debug(res.getBody());
        System.debug(res.getStatusCode());
        System.debug(res.getBodyDocument());
   }
}