the better answer to the accepted one should be

create your custom div on the same hierarchy level as the existing field, then move your div inside it

the accepted answer is ok if you div is simple, but if you have a complex definition with domain definitions, readonly / visibility definitions and such you can keep your existing definition with my answer

<template id="website_sale" inherit_id="website.id_template" name="Name for your template "> 
 <xpath expr="//div[@id='odoo_default_row']" position="before">
   <div id="my_custom_div">
   </div>
</xpath>
 <xpath expr="//div[@id='my_custom_div']" position="inside">
   <xpath expr="//div[@id='odoo_default_row']" position="move"/>
</xpath>
</template>

You can replace a default element with a custom element or you can add a custom element to an default element. If you want to add a default element to a custom element, then you have to replace the default element with an element, which contains your custom element and the default element.