dropdown in chrome extension with js

I’ve been looking everywhere and can’t seem to make this work , when am trying to add a dropdown in chrome extension for my project,it shows error. I tried with all functions(addEventListener , #Id option:selected , onclick) there is no use.

Auto set an option in SelectBox ul li Drop Down menu.[JAVASCRIPT]

I am new to JavaScript and I wish to auto set an option in a drop down menu while auto filling a form in background, using chrome extension.

I wish to code in Javascript only, no Jquery!

Here is the code of the drop down menu-

Please help, Thank You.