You can use the following shortcode to display a form field value in the notification email:
input_name is the name of the field in the form.
How to check the field name?
To check the field name you can right click on the field and use the inspect element in your browser. You will see the HTML output where the input has a name attribute. This value is a field name.
Default field names:
Any field generated in the form customizer will get a unique name which should be checked with the browser inspect element feature.