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This restricts the input for users to a set of choices as shown in the screenshot below.
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Be sure to change the name of the payment method as well as the name of the customer field. Set required as appropriate. Be sure to not change the fieldType. The options describe the possible selections for the user. label is what is shown to the user. value is what is taken as the final option and is thus shown in the service panel, documents and CSVs.
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