When you want to trigger a tag upon form submission in GTM and you are installing GTM on your Unbounce page using Script Manager, you could set up a custom event using gtm.formSubmit event. Follow our documentation: How Do I Integrate Google Tag Manager With Unbounce?

In the case where you have GTM installed manually, on the variant level, you would need to set up your triggers configuration as the default Form Submission in GTM as you can see below: