Customise message.phtml pop up message magento 2

Question

I want to change the message.phtml template file which renders a pop up message below on product page.

Im not sure where in the KO.js binding message html to change "My new message"

Here is the message.phtml

<?php

<div data-bind="scope: 'messages'">
    <div data-bind="foreach: { data: cookieMessages, as: 'message' }" class="messages">
        <div data-bind="attr: {
            class: 'message-' + message.type + ' ' + message.type + ' message',
            'data-ui-id': 'message-' + message.type
        }">
            <div data-bind="html: message.text"></div>
        </div>
    </div>
    <div data-bind="foreach: { data: messages().messages, as: 'message' }" class="messages">
        <div data-bind="attr: {
            class: 'message-' + message.type + ' ' + message.type + ' message',
            'data-ui-id': 'message-' + message.type
        }">
            <div data-bind="html: message.text"></div>
        </div>
    </div>
</div>
<script type="text/x-magento-init">
    {
        "*": {
            "Magento_Ui/js/core/app": {
                "components": {
                        "messages": {
                            "component": "Magento_Theme/js/view/messages"
                        }
                    }
                }
            }
    }
</script>

snippet of chrome browsers knockoutjs context extension

UPDATE AFTER CHANGES

Thanks for the help...I added this to my di.xml

<?php

namespace  {{Vendor}}\{{Module}}\Plugin;

use Magento\ConfigurableProduct\Model\Product\Type\Configurable; {

    class MagentoCaptchaValidation
    {
        public function afterGetSpecifyOptionMessage(Configurable $subject)
        {
            return __('Please select a size in order to place your order');
        }
    }
}

Error message received during the compile, why do i need this class MagentoCaptchaValidation

In PhpScanner.php line 183:
                                                                           
  Class {{Vendor}}\{{Module}}\Plugin\MagentoCaptchaValidation does not exist```

Answer 1

I just did a quick search for that string, and this is the only place it's showing up: vendor/magento/module-configurable-product/Model/Product/Type/Configurable::getSpecifyOptionMessage(). That seems to be where the text is ultimately coming from, and you could use a plugin to change it.

---Update---

For the plugin, you would just create an after plugin for getSpecifyOptionMessage(). So, in your custom module, you'd drop this in di.xml:

<type name="Magento\ConfigurableProduct\Model\Product\Type\Configurable">
   <plugin name="updateOptionMessageText" type="{{Vendor}}\{{Module}}\Plugin\UpdateOptionMessage"/>
</type>

And this into {{Vendor}}\{{Module}}\Plugin\UpdateOptionMessage:

<?php

namespace {{Vendor}}\{{Module}}\Plugin;

use Magento\ConfigurableProduct\Model\Product\Type\Configurable

class UpdateOptionMessage
{
    public function afterGetSpecifyOptionMessage(Configurable $subject)
    {
        return __('{{Whatever text you want}}');
    }
}