From Squarespace Help:
Use Code Blocks to add custom code to a page, blog post, sidebar, footer, or other content area. Code Blocks are good options if you want to embed third-party widgets or customize pages beyond what's possible with other blocks. You can also use Code Blocks to render HTML and Markdown or display code snippets.
On all plans, Code Blocks support plain text, HTML, Markdown, and CSS code surrounded by <style></style> tags.
When adding Acorn banner you will be using a HTML code snippet like:
<a href="<a href="https://www.acornfinance.com/apply?d=JL92Z" target="_blank" rel="nofollow noopener noreferrer">https://www.acornfinance.com/apply?d=JL92Z</a>" target="_blank" rel="noopener noreferrer"><img id="acornBanner" style="margin: 10px; padding: 1px; border: 1px solid black; border-radius: 5px;" src="<a href="https://fs.acornfinance.com/banners/acorn-finance-banner-easy-payment-options-vertical-medium.png" target="_blank" rel="nofollow noopener noreferrer">https://fs.acornfinance.com/banners/acorn-finance-banner-easy-payment-options-vertical-medium.png</a>" alt="Acorn Finance apply and get affordable payment options from multiple lenders" longdesc="false"></a>