Hi @Maxim Velichkin,
Thanks for your question.
In order to open sidebar using a button, add a Show page activity from the toolbox. In the "Show page" action, select the page that contains the sidebar you want to open.
In the "Properties" tab of the "Show page" action, select the "Sidebar" option in the "Open in" field.
PS: Note: The page that contains the sidebar has the "Has sidebar" option enabled in its properties.
When you click the button, it should open the sidebar.
In case this solves your question, please mark this answer as accepted.
Thanks,
Amit G