data-protection/website_contact_extend/views/website_contact.xml

<?xml version="1.0" encoding="utf-8"?>

<odoo>
    <template id="contactus_form_inherited" name="Contact Form" inherit_id="website_crm.contactus_form" customize_show="True">
        <xpath expr="//div[@class='form-group form-field o_website_form_required_custom'][last()]" position="after">
            <div class="form-group form-field o_website_form_required_custom">
                <label class="col-md-3 col-sm-4 control-label" for="contact_type">In the future I want to be contacted by</label>
                <div class="col-md-7 col-sm-8">
                    <div class="checkbox>"><label><input type="checkbox" name="letter_contact" value="letter_contact"/> Contact by Letter </label></div>
                    <div class="checkbox>"><label><input type="checkbox" name="email_contact" value="email_contact"/> Contact by E-Mail </label></div>
                    <div class="checkbox>"><label><input type="checkbox" name="phone_contact" value="phone_contact"/> Contact by Phone </label></div>
                    <!--<select class="form-control" name="contact_type" id="contact_type">
                        <option value="no_contact">I do not want to be contacted.</option>
                        <option value="email_contact">I only want to be contacted by Email.</option>
                        <option value="phone_contact">I only want to be contacted by Phone.</option>
                        <option value="email_phone_contact">You can contact me by Email or Phone.</option>
                    </select>-->
                </div>
            </div>
            <div class="form-group form-field">
                <label class="col-md-3 col-sm-4 control-label" for="contact_type">Information request concerning data about the personally</label>
                <div class="col-md-7 col-sm-8">
                    <input type="checkbox" name="send_mail" value="send_mail"/>
                </div>
            </div>
        </xpath>
    </template>

</odoo>