Unfortunately, out of the box, you won't be able to do this with radio buttons. If you want radio buttons, you're going to need to custom dev work.
You're going to need to change your field to a "Select" field type in your form and then choose "Multiple Selections" and enter the values.
Radio buttons are a supported type of Marketo form widget.
Radios are, however, categorically the wrong UI element for this since they are mutually exclusive, allowing *one* choice from the set. There's no such thing as multi-select radios.
A checkbox set (Checkboxes widget type) is the common approach.
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Here's a peek at the options in the Form Editor for context-
Using the Checkboxes instead of Checkbox will allow you to make multiple selections.
Using Checkbox acts more like a radio button (where one selection un-selects the rest) but with the checkbox look instead of the radio.