How to add Dropdown menu in Squarespace

A. Single Dropdown Menu

#1. First, click on the Plus icon (+) in Not Linked or Main Navigation 

#2. Next, scroll down to the bottom and choose Dropdown

#3. Then, click ADD PAGE below the Dropdown to make your Dropdown menu items

B. Multi Columns Dropdown Menu

#1. First, you need to know this

#2. Use this code to Custom CSS box

div.header-nav-item:nth-child(2)>div {
    column-count: 2;
    column-gap: 5px;
}

#3. Result

C. Multi Levels Dropdown Menu

You can use Nested Menu Plugin