Yes, you can. Go to the Advanced section- Form tab.
Tick the option Add a JS script to your form. Paste the script URL in the text box that shows up. Use a reliable service to host your script files. This feature is available starting with the Platinum plan. Here is the 123FormBuilder features list.
If you need to track the numbers of users who submitted the form, simply add a script on the Thank-you Page.
Here is a list of the most frequently asked questions. For more FAQs, please browse through the FAQs page.