My name is Lizzy. I'm one of the Gurus on the support team at Shopify.
You could use an announcement bar or banner app to set something like this up on your store! The options listed below are the ones that I was able to find which specifically mention being able to place the banner anywhere, or have customization ability.
This page here has several more options if you don't find that any of the above will work for your business.
If you're more interested in hard coding something like this, you could post in the expert marketplace to find a developer experienced in Shopify who will be able to help! Feel free to let me know if you have any questions about this!
What sort of details are you looking to share in this banner? Is it going to be used to share information or as a call to action?