The following snippet will display a dropdown list in the checkout, and will display an extra text box for “Other” input if “Other” is selected.

On the Templates configuration page of your store's FoxyCart administration, under “Cart” look for “Add custom header or footer code to your website”, check the box if not checked, and add this code into the “Footer” text area.

{% if context == "checkout" %}
<script>
$("body").on("change", "#referral_source", function() {
    setTimeout(FC.checkout.renderAdditionalFields, 500);
});
</script>
{% endif %}

Next, and still on the configuration page, under “Checkout” look for “Add custom form fields to your checkout”, enable it, and paste the following into the text box within:

<div class="fc-form-group ">
    <div class="col-md-8 col-md-offset-3" data-fc-error-for="referral_source" data-fc-error-class="fc-alert-container--error">
        <div class="fc-input-group-container fc-input-group-container--checkbox fc-input-group-container--active">
            <label class="fc-input-group-container__title fc-input-group-container__title--forced fc-form-label">
                How did you find Us?… (required)
            </label>
            <div class="fc-form-group">
                <p>Referred by?</p>
                    <select name="referral_source" id="referral_source" class="fc-form-control" aria-required="true" data-fc-required >
                    <option value="Google" {% if referral_source == "Google" %}selected{% endif %}>Google</option>
                    <option value="Yahoo" {% if referral_source == "Yahoo" %}selected{% endif %}>Yahoo</option>
                    <option value="Bing" {% if referral_source == "Bing" %}selected{% endif %}>Bing</option>
                    <option value="Facebook" {% if referral_source == "Facebook" %}selected{% endif %}>Facebook</option>
                    <option value="Instagram" {% if referral_source == " Instagram " %}selected{% endif %}> Instagram </option>
                    <option value="Twitter" {% if referral_source == " Twitter" %}selected{% endif %}> Twitter</option>
                    <option value="Friend" {% if referral_source == "Friend" %}selected{% endif %}>Friend</option>
                    <option value="Amazon" {% if referral_source == "Amazon" %}selected{% endif %}>Amazon</option>
                    <option value="Email" {% if referral_source == "Email" %}selected{% endif %}>Email</option>
                    <option value="Returning Customer" {% if referral_source == "Returning Customer" %}selected{% endif %}>Returning Customer</option>
                    <option value="Other" {% if referral_source == "Other" %}selected{% endif %}>Other</option>
                    </select>
            </div>
            {% if referral_source == "Other" %}
                <div class="fc-form-group">
                    <input type="text"
                    id="other_text"
                    name="other_text"
                    placeholder="Please enter 'Other'"
                    class="fc-form-control fc-form-control--new-customer-password"
                    data-fc-required
                    value="{{ other_text }}"
                    />
                </div>
            {% endif %}
        </div>
    </div>
</div>