Execute Javascript Function from From

Execute Javascript Function from From

Sorry for this simple question but I can't find the answer anywhere...I'm
also a Javascript newbie with 0 experience so I'm really stuck.
Anyways, I have a function that is used with a simple button click:
<input type="button" value="Add Item" onclick="addToCartt('MODELNUMBER');">
I need to change that from the current setup where you can click and hit a
button and run the function with the ModelNumber argument into a form
where you enter the ModelNumber manually and hit return via an html form
and run the function addToCartt with the ModelNumber as the argument.
Thanks for any help