PS3.Extension.WriteString(0x3891f7b1 + (0x10 * numericUpDown1.Value), "Hello hurr durr");
That simple, make sure you use add a numericUpDown that is named the same as that above. Also, add a textBox and refer it instead of the string so you can type whatever you want. I am sure you knew that, though.
(Don't quote me on the PS3 function, I just whipped this up in the reply. i dont know if its .Extension or just .WriteString)