public class sfdcTOsnow{
    @Future(callout=true)
    public static void servicenowPost(Id CaseId){

        Case c = [Select Id, CaseNumber from Case where Id =: CaseId limit 1];
        Http http = new Http();
        HttpRequest req =  new HttpRequest();
        HttpResponse res = new HttpResponse();
        String username = 'admin';
        String password = 'Stech1234';
        Blob headerValue = Blob.valueOf(username + ':' + password);
        String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue);
        req.setHeader('Authorization', authorizationHeader);
        req.setEndpoint('https://dev62899.service-now.com/');
        req.setMethod('POST');
        req.setHeader('Content-Type', 'application/json');

        JSONGenerator gen = JSON.createGenerator(true);
        gen.writeStartObject();

        gen.writeStringField('number', c.CaseNumber);
        gen.writeStringField('caller_id','62826bf03710200044e0bfc8e5df1');
        gen.writeStringField('short_description','This is Test');
        gen.writeEndObject();

        String pretty = gen.getAsString();
        system.debug('---Pretty---'+pretty);

        req.setBody(pretty);
        res = http.send(req);

        system.debug('---Body---'+res.getBody());
    }
}

trigger servicenow on Case (after insert){
    sfdcTOsnow.servicenowPost(trigger.new[0].Id);
}

req.setEndpoint('https://dev62899.service-now.com/api/now/v1/table/incident');