Tag Action and Register a Goal conversion from wffm to Sitecore forms

It is pretty straight-forward in Sitecore Forms. You can achieve this using Submit action.

Open Sitecore Forms, in your form, drag-and-drop a submit button from the Form Elements pane (Structure section), if it is not already present.

Then,

1. select the Submit button
2. On the Form Elements pane, expand the Submit Actions section, click the Add button
3. Click on Trigger goal and browse to the goal definition item that you want to trigger on form submit.

Refer the below link for knowing all the available Submit actions OOTB : https://doc.sitecore.com/users/90/sitecore-experience-platform/en/working-with-submit-actions.html