Need to automatically add tag to customers who enter email in Shopify Forms

[Tom100]

How can I assign a tag [automatically] for website visitors who enter email in a form? Specifically Shopify Forms but I would like to do this is an 3rd party pop up form app too

[paul_n]

For Shopify Forms - you can use Metaobject entry created as the trigger and choose the form. 

Not sure about 3p forms - you need to ask the app owner