You can follow these steps to add banner to individual event page in Squarespace
#1. First, add this code to Code Injection > Footer
#2. Next, hover on Event Page > Click Gear icon
Click Page Header Code Injection
#3. Paste this code
<div data-wm-plugin="blog-post"></div>
#4. Use this code to Custom CSS box
/* Individual Event Banner */
.section-background.wm-blog-banner>div:not(.section-background-image) {
display: none;
}
#5. Result
The code will get Event Featured Image/Thumbnails and make it to banner image.
#6. To reduce banner image height, use this code to Custom CSS box
div.section-background.wm-blog-banner {
min-height: 50vh !important;
}