Open ninja-contact-form.min.js in Plugins/editor and add this snippet at the very end:

       window.NinjaSidebar && NinjaSidebar.showSidebar($(document))

Caution: Please use it wisely. Without additional conditions it will open slider each time and can annoy your visitor.

We recommend to use it on specific pages and with special condition. For example set cookie to check if the form has already been viewed by visitor. If so then don’t show it again. It will require from you some additional JS coding.