Can CP be used to automatically append code

Can I use custom properties to automatically append code onto the bottom of content? Will need to display in both article and blog_item mode. Looking at putting code onto the bottom of each article with an image that the user can click which will lead to an external URL.

The only things Custom Properties appends at the end of the article are the tags that have been assigned to that article.

Looking at putting code onto the bottom of each article with an image that the user can click which will lead to an external URL.

That sure looks like a task for a plugin. Possibly an existing one among the +8500 extensions for Joomla currently available.
The requirements are too sketchy (which image, which url , what kind of code, is it stored somewhere or it is inferred from the article, etc, etc) to give you any suggestion.

The image would just be one stored on the server ie. Click here button. The code would just be html which would be the same as href, image and image alt. the URL is also stored in the article params in SQL.

On a side note is it possible to display the tags in a table? So I could display say a 2x2 table with a different tag property in each box?

...code would just be html...

I'm second guessing myself about the need for a plugin. Why don't you just write that html code at the end of the article?

Cptags as a table:
No tables. But you can have your table-like layout with a modifications to the CSS stylesheet.

The html code would be at the end of every srticle and I add around 20 per day so trying to avoid copying and pasting each time.

Any links for the css code in tables off the top of your head?

You should look here:
extensions.joomla.org/extensions/edition/custom-code-in-content

Any links for the css code in tables off the top of your head?

There's no standard CSS code for that. It strictly depends on the graphic layout you have to implement (colors, spacing, fonts, elements placement, etc).