String resp = 'response : ';
        HttpRequest req = new HttpRequest();
        JSONGenerator gen = JSON.createGenerator(false);
        JSONGenerator gen = JSON.createGenerator(false);
  		String  url = 'http://api.xxxxx.com/xx/xxx/xxx';
        
        	req.setEndpoint(url);
			String reqJSON;
                        gen.writeStartObject();
                        gen.writeStringField ('transaction_id', txnID);
                        gen.writeEndObject();
                        
                        // Get the JSON string.
                        reqJSON = gen.getAsString();        
	        req.setBody(reqJSON);
			req.setEndpoint (url);
	        req.setHeader('Authorization', 'Token xxxxxxxxxxxxxxxxxxxxxxxx');
        
                        req.setMethod ('GET');
                        req.setHeader('Content-Type', 'application/json');
                        req.setHeader('Accept', 'application/json');
                        req.setBody(reqJSON);
                        req.setTimeout(60000);
                      // Get response

        httpResponse res = new http().send(req);
		resp = resp + ' : ' + res.getBody() + ' - '  + res.toString();
	    ApexPages.addMessage(new ApexPages.Message( ApexPages.Severity.info,resp));

response : : - System.HttpResponse[Status=Moved Permanently, StatusCode=301]

• I have checked the API , its working fine on Postman and other platform.
• I have added the api url on Remote Sites.