My response about setting the values directly in the insert server behavior would only apply if you are using DataAssist for the insert or updates. Based on your reply I assume you are using the regular built in DW insert that is created by default with Security Assist.
In this scenario you will need to have the hidden form elements in place with values to get them inserted correctly.
To get the value for the date formatted the way you would like it you can use the date function along with the time function like this:
<input type="hidden" name="created_dt" id="created_dt" value="<?php echo date("Y-m-d H:i:s", time()); ?>" />
The value of this form field will be the current date and time in the format you referenced:
You can find out more about this awesome formatting function here: