Why not try to use this library?
I confirmed that this library works fine and is easy to use.
The simple example clearly shows how to send a message.
Though I have not tested the library with physical buttons, the code you want will be like the following:
if (buttonState == HIGH) {
Serial.println("connecting ...");
if (twitter.post(msg)) {
int status = twitter.wait();
if (status == 200) {
Serial.println("OK.");
} else {
Serial.print("failed : code ");
Serial.println(status);
}
} else {
Serial.println("connection failed.");
}
delay(1000); //prevent multiple sending at a time
}