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Add to Home > Design > Custom CSS

.Header-nav-item[href="/events"] {
    position: relative;
}
.Header-nav-item[href="/events"]:after {
    content: "";
    background: red;
    width: 5px;
    height: 5px;
    display: block;
    border-radius: 50%;
    position: absolute;
    right: 0;
    top: 0;
}

21 hours ago, siweisong said:

plugins or codes

I found this ewebdesign article, it is not Squarespace specific but it outlines the process. If you have some coding knowledge this might be the ticket.