Add a Heading above Add-Ons Product

Use this code to the Custom CSS box

section.pdp-product-add-ons:before {
    content: "Add Ons Product";
    display: block;
    font-size: 30px;
}
section.pdp-product-add-ons>div:first-child {
    margin-top: 10px !important;
}

Result: