Validation in React Hook Form to make sure at least one checkbox is checked?

I am working on a form in which I have a section of it that needs to have at least one of the checkboxes checked. I am using ReactJS and a React Hook Form.

Here is the checkbox section of code within my render function:

{/* Checkbox group. User must select at least one medium. */}
<label><b>Medium (check all that apply): *</b></label>
<div>
 <label>
  <input type="checkbox" name="medium" value="Design & Illustration" onChange="validateMedium();"/><span>Design & Illustration</span>
 </label>
</div>
<div>
 <label>
  <input type="checkbox" name="medium" value="Digital Art" onChange="validateMedium();"/><span>Digital Art</span>
 </label>
</div>
<div>
 <label>
  <input type="checkbox" name="medium" value="Drawing" onChange="validateMedium();"/><span>Drawing</span>
 </label>
</div>
<div>
 <label>
  <input type="checkbox" name="medium" value="Painting & Mixed Media" onChange="validateMedium();"/><span>Painting & Mixed Media</span>
 </label>
</div>
<div>
 <label>
  <input type="checkbox" name="medium" value="Photography" onChange="validateMedium();"/><span>Photography</span>
 </label>
</div>

Here is the function I am trying to write to validate the checkbox group:

function validateMedium() {
    var mediumCheckboxes = document.getElementsByName("medium");
    var okay = false;

    for (var i = 0, len = mediumCheckboxes.length; i < len; i++) {
        if (mediumCheckboxes[i].checked) {
            okay = true;
            break;
        }
    }

    if (okay) {
        alert("Thank you");
    } else {
        alert("Please check at least one medium.");
    }
}

How do I integrate the validation function I've written specifically for the checkbox group on this form? If the user does not check at least one checkbox when the submit button is clicked, I would like the error to appear. I do not want to use jQuery.