If you need to set some value to a variable, you can call the input() function like this
$FEStopShort=Input("Enter Stop Price",Price,1);
Which will prompt you like this
The first parameter is the name of the window or the question. The second parameter is a value, in this case Price is where I clicked on the chart. You can use any variable as a predefined value or a static value. The last entry - 1 means how many lines of the prompt window you want to display. A prompt with the value being 5 looks like this
You can put numeric or text strings to assign them to variables like this. A practical use of this feature is in the below post