If you navigate to Admin > Forms, you can create new forms with custom fields.
In the Add Custom Field pop-up, you can either use a Saved Field or create a New Field. If the field you want to use is in the Saved Fields, then select that one by navigating through the drop down. Then click “Add Custom Field”.
If not, you can create a new one by clicking “New Field” and then select the field type you would like to use for your form. Then click “Add Custom Field”.