• If you are able to consume the API using cURL or Postman, thats a good news. You can now replicate similar methods in Salesforce.
• Though Apex and Java have similarities, Java code will not work in Apex.
• If its a Basic Authentication, you can try using a code like below,

public class WebServiceCallout {
    
    @future(callout=true)
    public static void sendCalloutREST(String jsonString){
        try{
            String endPointURL = 'https://samplesite.com/samplerest';
            String userName = 'samuser';
            String password = 'Sam@789';
            
            // Specify the required user name and password to access the endpoint
            // As well as the header and header information
            Blob headerValue = Blob.valueOf(userName + ':' + password);
            String authorizationHeader = 'BASIC ' +
                EncodingUtil.base64Encode(headerValue);
            
            Httprequest request = new HttpRequest();
            Http http = new Http();
            
            request.setMethod('POST');
            request.setEndpoint(endPointURL);
            request.setHeader('Content-Type', 'application/json');
            // Header info with remote server user name and password
            request.setHeader('Authorization', authorizationHeader);
            //Check the client certificate
            request.setClientCertificateName('Sample-Rest-Self-Signed');
            // timeout in milliseconds       
            request.setTimeout(120000);
            request.setBody(jsonString);          
            
            //Making call to external REST API
            HttpResponse response = http.send(request);  
            
            System.debug('responseBody: '+response.getBody());
        }catch(Exception e){
            System.debug('Error::'+e.getMessage());
        }
        
    }
}