How to execute/inject a script in a WebBrowser?

Hi everyone, i want to send a custom function programatically to my webbrowser how can i do that?

Example of what i would like to do:

WebBrowser1.ExecuteScript(“HERE THE SCRIPT”)

I tried CallAsync but  it seems that it only works for calling existigng functions in the code, wich i dont want!

(I dont want to execute an existing function, i want to INJECT a SCRIPT and execute it immediately in the web browser!

Thanks everyone!