Dynamic form fields work in tandem with single select form fields such as radio buttons or select "dropdowns". 

In the example below when the "Other" option is selected the "Please describe" field appears:

First, add the field that you want to display when a certain option is selected on the firm. In the example above this field is a custom field called "please describe".

Next, add a single select field (radio buttons or dropdown) and use the "Dynamic behaviour" setting in the sidebar to choose which selection is to make the dynamic field to show up on the form.

Related: read more about working with forms such as styles, standard and custom fields, display options, subscriptions, etc.