So, this is how I did it.
I am using systeme for my email (the free version).
I have created a new funnel especially for newsletter OPT-IN.
After editting it and saving it, you have the option of copy the html code.
I have copied the code and pasted in the Site Settings / Newsletter box.
Ta daaa! I have it now. ItΒ΄s ugly because I used a pre-existing template but you can make it from scratch if you want something ...well, not so green :))