Http http = new Http();
HttpRequest request = new HttpRequest();
//String Endpoint = 'https://login.salesforce.com/services/oauth2/token';
request.setEndpoint('callout:CIPHR_Kimble/services/oauth2/token');
//request.setEndpoint(Endpoint);
request.setMethod('POST');

String ClientId = 'XXXXXXXXXXXXXXXXXXXXXXXXXXX';
String ClientSecret = '11111111111111111';
String username = 'User@example.com';
String password = 'Password+SecurityCode';
request.setHeader('Content-Type', 'application/x-www-form-urlencoded');
request.setBody('grant_type=password' +
              '&client_id=' + ClientId +
              '&client_secret=' + ClientSecret +
              '&username=' + username +
              '&password=' + password
           );
HttpResponse response = http.send(request);
// Parse the JSON response
String Access_Token;
system.debug('body:'+response.getBody());
JSONParser parser = JSON.createParser(response.getBody());
while (parser.nextToken() != null) {
    if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && (parser.getText() == 'access_token')) {
        parser.nextToken();
        Access_Token = parser.getText();
    }
}
system.debug('access token'+Access_Token);

String sfdcURL = URL.getSalesforceBaseUrl().toExternalForm();
String restAPIURL = sfdcURL + '/services/apexrest/KimbleOne/v1.0/Import/UpdateResourceCalendar';
String AccessToken = Access_Token;

HttpRequest httpRequest = new HttpRequest();
//httpRequest.setEndpoint(restAPIURL);
httpRequest.setEndpoint('callout:CIPHR_Kimble/services/apexrest/KimbleOne/v1.0/Import/UpdateResourceCalendar');
httpRequest.setMethod('POST');
httprequest.setHeader('Content-Type', 'application/json');
httprequest.setHeader('Accept','application/json');
httpRequest.setHeader('Authorization', 'OAuth '+Access_Token);
httpRequest.setHeader('Authorization', 'Bearer '+Access_Token);

httpRequest.setBody('[{ "CIPHR_Id" : "7648_44573", "hours" : 2.40, "date" : "2018-07-18", "activity" : "a601X0000000030QAA", "resource" : "10187" }]');

System.debug('Access token '+AccessToken);
String responseString = '';
try {
    Http http1 = new Http();
    HttpResponse httpResponse = http1.send(httpRequest);
    System.debug('>> Response of payload >> '+httpResponse.getStatusCode());
    responseString = 'Status Code: '+httpResponse.getStatusCode()+' Response: '+httpResponse.getBody();
} catch(Exception e) {
    System.debug('ERROR: '+ e.getMessage());
    responseString = e.getMessage();
}
System.debug(' ** response ** : ' + responseString );