Hi @marcomorello,
You can use WPCode to easily add code to the head are of your page and at the beginning of the body tag.
You can do so either by using the Header & Footer area and directly pasting the code in there or creating code snippets set to Site Wide Header and Site Wide Footer.
Regarding inserting code between the head and body elements that is not directly possible with the options described above simply because that would not be valid HTML. Most tracking scripts require adding a script in the head area and another piece of code at the begging of the body tag.
While it would likely be possible to insert code between the head and body tags with our “Insert Anywhere” feature in the premium version of WPCode I would advise against it.
Let me know if I can provide more info.