Hii Aman,
I have created a sample code to make a callout in another org where you have created your API.
You only need to replace the variables in my code.
Here is the code:-


public class CalloutExampleToCreateMultipleContacts {
    public static final String CONSUMER_Key = ''; // paste your client id here
    public static final String CONSUMER_SECRET = '';// paste the client secret here
    public static final String USERNAME = 'deepanshu.gupta@cloudanalogy.com'; // paste your your name here
    public static final String PASSWORD = '';//paste the salesforce password and the security token
    

    // pass any list of contact here and call this method to perform the callout
    public static void insertContact(List<Contact> contactObjList){
        AccessTokenWrapper accessTokenWrapperObj = genrateToken(); //this is the wrapper class of token generation 
        system.debug('--access token->'+accessTokenWrapperObj);
        if(accessTokenWrapperObj != null && accessTokenWrapperObj.access_token != null){
            String endpoint = 'https://cloudanalogy123-dev-ed.my.salesforce.com/services/apexrest/createContact22/';
            String requestBody = JSON.serialize(contactObjList);
            HTTP http = new HTTP();
            HttpRequest request = new HttpRequest();
            request.setBody(requestBody);
            request.setMethod('POST');
            request.setHeader('Authorization', 'Bearer '+accessTokenWrapperObj.access_token);
            request.setHeader('Content-type','application/json');
            request.setHeader('Accept','application/json');
            request.setEndpoint(endpoint);
            HttpResponse response = http.send(request);
            System.debug('Status code:'+response.getStatusCode()+'==>'+response.getBody());
            
            
        }
    }
    
    // function to get the access token
    public static AccessTokenWrapper genrateToken(){
        String requestBody = 'grant_type=password&client_id='+CONSUMER_Key+'&client_secret='+CONSUMER_SECRET+'&username='+USERNAME+'&password='+PASSWORD;
        String endpoint = 'https://login.salesforce.com/services/oauth2/token';
        HTTP http = new HTTP();
        HttpRequest request = new HttpRequest();
        request.setBody(requestBody);
        request.setMethod('POST');
        request.setEndpoint(endpoint);
        
        HttpResponse response = http.send(request);
        System.debug('response==>'+response.getBody()+'Status code:'+response.getStatusCode());
        if(response.getStatusCode() == 200){
            system.debug('');
            return (AccessTokenWrapper)System.JSON.deserialize(response.getBody(), AccessTokenWrapper.class);
        }else{
            return null;
        }
        
    }
    
    // wrapper class to store the access token values
    public class AccessTokenWrapper{
        public string access_token;
        public string instance_url;
        public string id;
        public string token_type;
        public string issued_at;
        public string signature;
        
        
    } 
}
 


if you having a problem then Ask me 
Please mark Best Answer If Your Problem Is Solved

thanks, Regards,
Malika Pathak