zulip/web/templates/settings/edit_outgoing_webhook_service.hbs

<div class="input-group">
    <label for="edit_service_base_url">{{t "Endpoint URL" }}</label>
    <input id="edit_service_base_url" type="text" name="service_payload_url" class="edit_service_base_url required" value="{{service.base_url}}" maxlength=2083 />
    <div><label for="edit_service_base_url" generated="true" class="text-error"></label></div>
</div>
<div class="input-group">
    <label for="edit_service_interface">{{t "Interface" }}</label>
    <select id="edit_service_interface" class="modal_select bootstrap-focus-style" name="service_interface">
        <option value="1">{{t "Generic" }}</option>
        <option value="2">{{t "Slack's outgoing webhooks" }}</option>
    </select>
</div>