Add hamburger menu button in Elementor

First of all you need a simple html widget and put the following in to it:

<button class="ws-navigation__button">
    <span class="ws-navigation__hamburger">
        <span class="ws-navigation__button-line"></span>
        <span class="ws-navigation__button-line"></span>
        <span class="ws-navigation__button-line"></span>
    </span>
    <span class="ws-navigation__cross">
        <span class="ws-navigation__button-line"></span>
        <span class="ws-navigation__button-line"></span>
    </span> 
</button>

…and use the custom css function in Elementor Pro:

selector {
    z-index: 99;
}

.ws-navigation__button {
    display: block;
    width: 40px;
    height: 40px;
    position: relative;
    border: none;
    padding: 0;
    background: none;
    cursor: pointer;
}

.ws-navigation__hamburger,
.ws-navigation__cross {
    display: block;
    width: 60%;
    height: 60%;
    position: absolute;
    top: 20%;
    left: 20%;
}

.ws-navigation__button-line {
    display: block;
    width: 100%;
    height: 2px;
    position: absolute;
    background-color: #000;
}

.ws-navigation__hamburger .ws-navigation__button-line:nth-child(2) {
    width: 75%;
}

.ws-navigation__hamburger .ws-navigation__button-line:nth-child(1),
.ws-navigation__hamburger .ws-navigation__button-line:nth-child(2),
.ws-navigation__hamburger .ws-navigation__button-line:nth-child(3) {
    right: 0;
    transform: translateY(-50%);
    -webkit-transform: translateY(-50%);
    transition: width 0.15s ease-in-out;
    -webkit-transition: width 0.15s ease-in-out;
}

.ws-navigation__hamburger .ws-navigation__button-line:nth-child(1) {
    top: calc(50% - 4px);
    transition-delay: 0.4s;
    -webkit-transition-delay: 0.4s;
}

.ws-navigation__hamburger .ws-navigation__button-line:nth-child(2) {
    top: 50%;
    transition-delay: 0.5s;
    -webkit-transition-delay: 0.5s;
}

.ws-navigation__hamburger .ws-navigation__button-line:nth-child(3) {
    top: calc(50% + 4px);
    transition-delay: 0.6s;
    -webkit-transition-delay: 0.6s;
}

.ws-navigation__button--clicked .ws-navigation__hamburger .ws-navigation__button-line {
    width: 0;
}

.ws-navigation__button--clicked .ws-navigation__hamburger .ws-navigation__button-line:nth-child(1) {
    transition-delay: 0.1s;
    -webkit-transition-delay: 0.1s;
}

.ws-navigation__button--clicked .ws-navigation__hamburger .ws-navigation__button-line:nth-child(2) {
    transition-delay: 0.2s;
    -webkit-transition-delay: 0.2s;
}

.ws-navigation__button--clicked .ws-navigation__hamburger .ws-navigation__button-line:nth-child(3) {
    transition-delay: 0.3s;
    -webkit-transition-delay: 0.3s;
}

.ws-navigation__cross {
    transform: rotate(45deg);
    -webkit-transform: rotate(45deg);
}

.ws-navigation__cross .ws-navigation__button-line:nth-child(1) {
    width: 0;
    top: 50%;
    left: 0;
    transform: translateY(-50%);
    -webkit-transform: translateY(-50%);
    transition: width 0.15s ease-in-out;
    -webkit-transition: width 0.15s ease-in-out;
    transition-delay: 0.1s;
    -webkit-transition-delay: 0.1s;
}

.ws-navigation__cross .ws-navigation__button-line:nth-child(2) {
    width: 2px;
    height: 0;
    top: 0;
    left: 50%;
    transform: translateX(-50%);
    -webkit-transform: translateX(-50%);
    transition: height 0.15s ease-in-out;
    -webkit-transition: height 0.15s ease-in-out;
    transition-delay: 0.2s;
    -webkit-transition-delay: 0.2s;
}

.ws-navigation__button--clicked .ws-navigation__cross .ws-navigation__button-line:nth-child(1) {
    width: 100%;
    transition-delay: 0.5s;
    -webkit-transition-delay: 0.5s;
}

.ws-navigation__button--clicked .ws-navigation__cross .ws-navigation__button-line:nth-child(2) {
    height: 100%;
    transition-delay: 0.6s;
    -webkit-transition-delay: 0.6s;
 }