How to customize Jenkins dashboard opening page?

Please take a look the home screenshot.

How to do that?

That’s the “description” of the top page. It can be configured from the “Add description” link on the right side of the page. It appears on only that page.

If the markdown formatter plugin is installed, then markdown can be used in those types of locations to create the hyperlinks, headings, and lists.

You can also add a system message that appears on all pages. I did that in my installation with configuration as code. You can also change the system message from “Manage Jenkins” → “Configure System”