At the beginning i would like to mention that i am a beginner in Odoo development so please try to be simple as much as you can.

To make my question easy i have uploaded an image of what i want and here is the image:

https://drive.google.com/open?id=1wtPk-FE7gs4W4vv267auWq1gxO0M0Mgg

I want to know how can i target a web client main element like the body element or the sidebar element with xpath or any other possible method to add/change attributes?

To be more specific, i want to know how to add attribute "onclick='method()'" to one of the side menu links and add script at the end of the page to define that method.

Also please if you have any links to lessons or tutorials that can help me with this topic, it is very appreciated if you leave its link :)