Thank you for contacting syncfusion support.
#1: how can I set by default an empty field to my Query Builder like a placeholder (empty data)?
We have checked your requirement and we suspect that you need to display the place holder value for DropDownList at initial rendering. And we would like to let you know that, if you haven’t referred the rule property, so the QueryBuilder renders as like in your provided screenshot. For your convenience, we have prepared the below sample based on your requirement.
Could you please check the above sample and get back to us with more details, if we misunderstood your requirement?
#2: how can I show the "+" button when my data is empty?
Before we provide solution for this, we would like to know your exact use case requirement about “adding ‘+’ button when data is empty”. Could you please let us know where you want to add plus button and when it should be added?
Vinoth Kumar S