Use confirm() as a condition to if?

When you compare a return value to true you shouldn't use return true, just true:

function RemoveProduct() {
  if (confirm("Poista?") == true) {
    return true;
  } else {
    return false;
  }
}

You don't even need to do the comparison, as the result from confirm is a boolean value:

function RemoveProduct() {
  if (confirm("Poista?")) {
    return true;
  } else {
    return false;
  }
}

And you don't even need the if statement, you can just return the result from confirm:

function RemoveProduct() {
  return confirm("Poista?");
}

Remember to use return when you use the function in an event. Example:

<input type="submit" onclick="return RemoveProduct();" />

But as far as I know, I can't use another brackets on the if sentence conditions?

There is nothing that prevents you from executing a function within an if condition. That said, I always get all the arguments to my conditional settled before the if, for clarity and readability.

Here is your code greatly simplified.

var confirmed = confirm('whatever');
return confirmed;

confirm() returns a boolean value and you can return that. Like so:

function RemoveProduct() {
    return confirm("Poista?");
}