I solved it adding _variable=value into plugin code (within wpcf7-redirect-script.js) file as detailed here:
// Build http query
if ( form.http_build_query ) {
temp_http_query = jQuery.param( event.detail.inputs, true );
http_query = temp_http_query.replace(new RegExp(‘\\+’, ‘g’), ‘%20’);
redirect_url = redirect_url + ‘?_variable=0&‘ + decodeURIComponent(http_query);
} else if ( form.http_build_query_selectively ) {
http_query = ‘?_variable=0&‘;
selective_fields = form.http_build_query_selectively_fields.split(‘ ‘).join(”);
event.detail.inputs.forEach( function(element, index) {
if ( selective_fields.indexOf( element.name ) != -1 ) {
http_query += element.name + ‘=’ + element.value + ‘&’;
}
});
http_query = http_query.slice(0, -1);
redirect_url = redirect_url + decodeURIComponent(http_query);
}