Here's how to create one of those text fields with HTML that the user's supposed to copy-and-paste into their page. It's not hard.

Create a form.

Add a Text Area. Give it a name, and set the rows and columns.

From the text area dialog, click on the "Advanced Properties..."

Click on the Javascript tab.

Add a property named "onclick" with the value of "this.focus(); this.select();".

Click OK.

Click OK.

NOTE: I found a serious problem - NVU's code reformatting will cause the html code to break within myspace, because NVU inserts newlines. To fix the problem, you have to save out the source, join all the lines, and upload the file manually.