Http http = new Http();
HttpRequest request = new HttpRequest();
request.setEndpoint('URL');
request.setMethod('POST');
request.setHeader('Authorization','Basic ' +'czZCaGRSa3F0MzpnWDFmQmF0M2JW');
request.setHeader('Content-Type','application/x-www-form-urlencoded');
request.setBody('grant_type=password&username=USERNAME&password=PASSWORD');

HttpResponse response = http.send(request);
// Parse the JSON response
if (response.getStatusCode() != 201) {
    System.debug('The status code returned was not expected: ' +
        response.getStatusCode() + ' ' + response.getStatus());
} else {
    System.debug(response.getBody());
}