javascript for filling text box with value from dropdown list

[GetBryan]

I need to fill a textbox with a date depending on the selection from a dropdown list on the form. How do I do this? I'm not having much luck getting this thing figured out. Any help would be greatly appreciated.

Here's what I have now that isn't working. I'll also have an else if statement to compare the other selected items.
The Dropdownlist field is called Grant Number and the Textbox is labeled Text4

The different values for the grant dropdown list are TR15-100, TR16-100 or TR17-100

var e= document.getElementById("Tracking Group").text;

var f= document.getElementById("Text4");

if (e="TR15-100"){

f.text = "9/1/2015-3/31/2017";

}

[try67]

Are you talking about a PDF form or a web-page?

[GetBryan]

This is a pdf form

[try67 · Accepted solution]

OK. In that case you can use this code as the custom calculation script of Text4:

var e = this.getField("Tracking Group").valueAsString;
if (e=="TR15-100"){
    event.value = "9/1/2015-3/31/2017";
} else event.value = "";

[GetBryan]

That worked great. Thank you. Try67 greatly appreciated.

[Bernd Alheit]

Your JavaScript code is for web pages.