Try to use the `title` attribute, like this:

    <button 
        id="checkout-now"
        type="button"
        class="action primary checkout"
        data-action="close"
        data-bind="attr: {href: checkoutNowUrl, title: checkoutNowTitle}"
    />

Here is example from the `Magento_Authorizenet` module:

> app/code/Magento/Authorizenet/view/frontend/web/template/payment/authorizenet-directpost.html

    <button data-role="review-save"
        type="submit"
        data-bind="
        attr: {title: $t('Place Order')},
        enable: (getCode() == isChecked()),
        click: placeOrder,
        css: {disabled: !isPlaceOrderActionAllowed()}
        "
        class="action primary checkout"
        disabled>
        <span data-bind="i18n: 'Place Order'"></span>
    </button>