how to change pricing with product variants with custom checkbox

Well, while using your plugin to create custom form and lets say I create a checkbox lets say if user presses this checkbox then it will increase price by 20 USD but I don’t see any option to make variant prices setup. because with common attributes which are by default in wordpress it works fine and there you can create variations but for these custom forms, we don’t have any option to set increased price if checked. Cna you guide how to achieve this?

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