Generally it's a bad idea to put your sketch in a loop waiting for a key press. Humans are very slow and your sketch can be doing useful stuff other than waiting for a human. You also don't need to check for NO_KEY before you decise what to do with the key value. Just pass every value on to the switch/case statement. If you want something special done when no key was pressed, you can make a 'case NO_KEY:'.
void setup () {
Serial.begin(9600);
lcd.begin(16,2);
lcd.print ("HELLLO"); // Dispay "HELLO" until the '1' key is pressed.
}
void loop()
{
customkey = customKeypad.getKey();
switch(customkey) {
case '1':
lcd.clear();
lcd.setCursor(9,0);
lcd.print("WELCOME");
break;
}
}