Bridge between pbiviz and Power BI Desktop

Hello, 

I'm trying to understand the link between HTML+JS+CSS and Power BI Desktop. 

I'm interested in developing a custom chart, where a developer could help me by using a combination of HTML, JS and CSS. What i don't understand is how to add his chart to Power BI Desktop. As far as i've read online, it looks like I need a pbiviz file, but I don't know what instructions to send over to my developer. 

Could someone please guide me towards a direction?

Thank you!