Custom fields are found on the Forms & Fields page.
Field IDs are unique snippets of text that closely match or exactly match the name of a field. Note: If you update a field, the ID will remain the same.
If you use numbers in your field name, they will be omitted from the value. This could make all your values the same if you create fields like "alpha 1" and "alpha 2" both of the values would be "alpha" and could lead to mishaps when working with your data.
You can find a field ID by navigating to the Fields & Forms page, clicking on a field, and copying the value or text after data-ms-member. Your screen may look different depending on your website builder. For example, you may see something like data-ms-member="email". In this example, email is the field ID.
If you don't have any custom fields yet, you'll want to hit that button that says Add custom fields.