I've also used the solution eric linked to because I needed to have default text for the Description field in one of my calendars to be set with specific formatted text.
He's right about the textarea as the selector, use that instead of 'input'.
You'll want something like this:
function setTextAreaFromFieldName(fieldName, value) {
if (value == undefined) return;
var theInput = getTagFromIdentifierAndTitle("textarea","TextField",fieldName);
theInput.value=value;
The text I needed had to be blue and bolded.
Here's what I used:
setTextAreaFromFieldName("Description",('<font color="#0000ff"><b>Content:</b></font>
<font color="#0000ff"><b>Class Highlights:</b></font>
<font color="#0000ff"><b>Target and Recommended Audience:</b></font>
<font color="#0000ff"><b>Class Dates/Times:</b></font>
<font color="#0000ff"><b>Sign Up Sheet:</b></font>
<font color="#0000ff"><b>Class Length:</b></font>
'));