If you are in the new builder, and either add the ‘Form’ element to the landing page, or click on the existing ‘form’ element, the right sidebar to configure the form fields will appear, where you can add a field. The field dropdown will start with ‘Standard’ fields, and then roll into ‘Custom’ fields, allowing you to add any of the custom fields you have created.
Here is a quick visual I recorded. I just started a new landing page from scratch, and add the form element, to show how to select and add my custom fields. https://www.screencast.com/t/7gKe9SvjMj