Hi Mariah,
In Dynamics Marketing, we can accomplish it with javascript.
The script will execute after form is rendered.
MsCrmMkt.MsCrmFormLoader.on('afterFormLoad', function (event) {
var radiobtnList = document.querySelectorAll('input[type="radio"]');
for (var i = 0; i < radiobtnList.length; i ) {
radiobtnList[i].addEventListener('change', function () {
if (this.checked) {
document.getElementsByTagName("input")[0].value = this.nextSibling.innerText;
}
})
}
})
In my marketing form, the Topic field is the first field, so index number of document.getElementsByTagName("input")[0.value is 0, please adjust the number based on your environment.

Result:
The topic field of new lead has been set to text of checked radio button option.

Regards,
Clofly