How can I activate Collapse Menu in WordPress?

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You have to trigger the click event. Try,

jQuery(document).ready(function() { jQuery("#collapse-menu").trigger('click'); });

Update:
Use the following code to check if menu is already folded, if not trigger click event to fold it.

jQuery(document).ready(function() {
    if ( !$(document.body).hasClass('folded') ) {
        jQuery("#collapse-menu").trigger('click');
    }
});