There are actually different things that people can do in order to increase their webinar engagement. One of the most simple things to do is to create a q&a. When you allow people to answer some questions, they will be inclined to answer, especially if they find the questions interesting. You can also do a live chat just in case they have some inquiries that you would like to clear up soon.
You can also do some polls that will give you an idea of what people who are watching the webinar would like to see next. The more interested they are, the higher the chances that they will try to engage with you.