WordPress Plugin- How to Insert Html&JS code in specific Page and specific Div

As you want to insert the button on only one/few specific pages you can simply create a shortcode[Reference] and insert it on specific page/pages.

Your code will be something like:

function hook_html(){
     //function body
}
add_shortcode( 'dropbox_button', 'hook_html' );

And then just edit the specific pages and add the shortcode [dropbox_button] to them, you can also choose to add this in any section of the page that you want.