Follow this instructions to include the customers text into your order confirmation email.
Open the store settings and click on „Notifications„.

In the next step click on Order Confirmation:

In the last step you have to copy and paste the following code into your email template.
Search the position like in the example and paste the code:

{% assign property_size = line.properties | size %}
{% if property_size > 0 %} 
    {% for p in line.properties %}
        {% if p.first contains "_datepickerUniqueProperty_" %}
            <span style="font-size: .875em; color: #999;">{{ p.first }}: {{ p.last }} </span><br>
        {% endif %}
    {% endfor %}
{% endif %}