Input the text from the text box in other program textbox

Quote:

SendKey but htat is sending the specify text and my text will change

SendKeys.Send[^] sends whatever string you you pass it: the examples use a fixed string becasue they are just that: examples of how to use it. They are not explicit instructions on how to do exactly what your task requires, they are they to give you the idea how to use the method.
Wherever a method accepts a string in C#, it can accept a fixed string:

SendKeys.Send("Hello");

Or a variable containing a reference to a fixed string:

string aVariableName = "Hello " + myTextBox.Text;
SendKeys.Send(aVariableName);