The sketch I posted worked the last time I tried it. No modification necessary (apart from the recipient's number, and you may need to swap Rx / Tx over).
I've not investigated the library to see what the Reset connection does. I can't recall ever using it myself.
To check your connections upload a Serial relay / repeater sketch to the Mega. This is simply to send what is received from Serial to Serial1, and vice versa. There are numerous versions around, just do a search and pick one. Once up and running send AT to the SIM800L, which should respond OK. Once you have got that working you can move on to uploading the sketch to send a message.
The format of the recipient's number is the plus symbol, the international dialling code and the the actual subscriber number without the leading zero. So for Italy, for instance, it would be + 39 777712345 (without the spaces)