Utility class for an HttpURLConnection
If you would pass in your paramaters at sendPost(), you could reuse your created object for other URLs and Parameters, and would not need a specific constructor.

Generally, calling a Method in a constructor is defying the purpose of a constructor - to initialize an object. When rereading your code 2 months later, you would be wondering, where you'd be calling the actual POST.

Clean Code wise, it's generally a bad sign if you create subsections in your method using comments, such as //add reuqest header. Create seperate private methods in your class just for doing that. Your code will be more readable and reusable.

Exceptions; use them! Throw detailed Exceptions and declare them as such. So you can react in your other code to that exception.

String USER_AGENT = "Mozilla/5.0"; this should be either declared as private static final in your class, because when introducing other methods such as sendGet you'd be using the same variable again.

Generally, don't use System.out.println other than in very early stages of your code. Create a Response class, where you set the things such as the response code or others and return this, don't just write it to the console, you can't use it and your console is going to be littered with such output everywhere in your program, which makes it difficult to debug.

You could generalize the method creating your params String as well, so you could pass in an arbirtrary amount of parameters and it would serialize these parameters just right.