My custom field isn't showing up in the form editor.

Where it is actually made

CRM, then Contacts. Click any contact in the list, then Add new custom field. It takes a Name and a Unique key, then Save.

systeme.io's own note puts it plainly: custom fields are created from the contacts page in your dashboard, and once created they appear automatically in the form settings.

Where it shows up afterwards, and that is two places

With a Form input element: drag it onto the page, click it to open its settings, and the field you made is in the Input type list.

Inside a Form element it is somewhere else. systeme.io says custom fields appear automatically in the form settings under Custom fields. Fields are added from the plus button in the Form's Elements tab, after scrolling down.

Two elements, and they are not the same

A Form input is a single field placed on the page by itself, added separately from the button beside it. A Form is a container grouping input fields, checkboxes and a submit button together, which systeme.io says lets you reliably build multiple independent forms on the same page.

Saving the page goes one way

The warning at the end of systeme.io's article on adding a custom form input field: once the changes are saved, the old version of the page cannot be restored.