There may well be a prettier way to do this, but here is a quick way out in case you need it:
First, put the script on your server, someplace it’s easy to find. I usually have a “scripts” folder in my sites/all folder.
Second, when do your post, set your input filter to PHP
Third, do a php include where you’d like to place the script. Something like this:
<script> available without filtering would be very foolish security wise. Unfiltered js in a webform makes for a very insecure setup.