// download the dll here: http://fx1.net/inputbox.dll / see also http://forum.mql4.com/40641
#import "inputbox.dll"
   string InputBox(string prompt, string title, string default_value);
#import

usage:
string defaultvalue = "MY TEXT";
string userInput    = InputBox("MESSAGE1","TITLE MESSAGE 2",defaultvalue);

if you want to deal with numbers (int/double) you will have to convert them with e.g. StrToDouble()