How to open Mobile Menu on default

To make mobile menu open on default, you can use this code to Custom CSS box

body:not(.header--menu-open) {
.header-menu {
    opacity: 1 !important;
    visibility: visible !important;
    bottom: unset !important;
    height: 50vh;
}
.burger-inner .top-bun {
    transform: translatex(3.5px) rotate(-135deg);
    width: 28px;
}
.burger-inner .bottom-bun {
    transform: translatex(3.5px) rotate(135deg);
    width: 28px;
}}
body.header--menu-open {
.header-menu {
    opacity: 0 !important;
    visibility: hidden !important;
}
.top-bun {
    transform: translatey(-5.5px) !important;
}
.bottom-bun {
    transform: translatey(5.5px) !important;
}}