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I checked one of your forms and I can see the "Fetch Data from a Text Field" does not appear indeed, but this is because it requires a text field to be placed in the form to be included, other fields or widgets are not supported here, this is why you cannot find this option in your payment wizard.
You will need to add a text field to your form in order to get the option showing in your payment section, so you can select it as custom field.
Hope this helps.