Instapage allows running javascript code just after the form is successfully submitted. To achieve that, just add the JavaScript code in your Page settings - JavaScript, in the Header section:
Inside the "instapageFormSubmitSuccess" function where you have the text
// here is your code to execute
// form variable holds submitted form
You will need to add in your own code, like, for example, an event tracking code or a Facebook pixel.
NOTE: This function can only be present on the page once, regardless of how many forms you have or how many actions you want to trigger after form submission. In case you have multiple snippets that need to fire after form submission, you must add all of them under the same Form Submit Callback function. If the snippets are not added under the same Form Submit Callback script, only the last function in the source code will be triggered.