I have developed a payment method for Magento 2. I would like the “send order” button (“invia ordine”) to be disabled in checkout (or send an error message) if the customer does not select an item in the options menu of my method (“seleziona”).

/app/code/Infocurci/Pagamentodilazionato/view/frontend/web/template/payment/pagamentodilazionato.html
<div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked())}">
<div class="payment-method-title field choice">
<input type="radio"
name="payment(method)"
class="radio"
data-bind="attr: {'id': getCode()}, value: getCode(), checked: isChecked, click: selectPaymentMethod, visible: isRadioButtonVisible()"/>
<label data-bind="attr: {'for': getCode()}" class="label"><span data-bind="text: getTitle()"></span></label>
</div>
<div class="payment-method-content">
<div>
<select name="payment(condizionipagamento)" class="select input-text required-entry"
data-bind="
attr: {id: getCode()+'_condizionipagamento'},
options: getCardList(),
optionsValue: 'value',
optionsText: 'type',
optionsCaption: $t('--Please Select--'),
">
</select>
</div>
<!-- ko foreach: getRegion('messages') -->
<!-- ko template: getTemplate() --><!-- /ko -->
<!--/ko-->
<div class="payment-method-billing-address">
<!-- ko foreach: $parent.getRegion(getBillingAddressFormName()) -->
<!-- ko template: getTemplate() --><!-- /ko -->
<!--/ko-->
</div>
<div class="checkout-agreements-block">
<!-- ko foreach: $parent.getRegion('before-place-order') -->
<!-- ko template: getTemplate() --><!-- /ko -->
<!--/ko-->
</div>
<div class="actions-toolbar">
<div class="primary">
<button class="action primary checkout"
type="submit"
data-bind="
click: placeOrder,
attr: {title: $t('Place Order')},
css: {disabled: !isPlaceOrderActionAllowed()},
enable: (getCode() == isChecked())
"
disabled>
<span data-bind="i18n: 'Place Order'"></span>
</button>
</div>
</div>
</div>