I'm new to the infrared World, and I'm using an Arduino with "Ken Shirriff" Library "IRremote".

I have successfully created a small code to send and receive infrared signals between 2 Arduinos. My problem is I need to send custom codes using irsend.sendRaw(RawArray, ArrayLength, 38).

Lets say I want to send this HEX: A12345
That can converted to the Binary: 101000010010001101000101

I know that from the Binary I can create the Raw codes, but that's where I'm stuck. I have already chosen my "0", "1", and "space" values, respectively "500", "1000" and "500", so:
Binary 1 = Raw code {500, 1000}
Binary 0 = Raw code {500, 500}

With that I can create the array: RawArray[48] = {500, 1000, 500, 500, 500, 1000, 500, 500, 500, 500, 500, 500, 500, 500, 500, 1000, 500, 500, 500, 500, 500, 1000, 500, 500, 500, 500, 500, 500, 500, 1000, 500, 1000, 500, 500, 500, 1000, 500, 500, 500, 500, 500, 500, 500, 1000, 500, 500, 500, 1000,}

Now, I'm missing the starter values from my Array, that are supposed to give the "settings" of my transmission, I belive they are called HDD_MARK and HDR_SPACE. I find so many different explanations, that go on completely different approaches that I'm completely lost at this point.

Can someone explain me how to calculate and include those values?

I think you need to go to the Arduino forum, IR codes on the forum are CIR codes ( consumer Infra Red) and the codes are created by manufactures, what we do here is look for the discrete codes that do not appear on the Factory remote.

Most are in the Pronto Raw hex format .. if you google "Barry Gordon Pronto Hex" you will get a complete guide to this.