@simi98 Thank you for contacting support. If you place the ad banner script into the Snippet / Code textarea and select Location as Header of the HFCM plugin, the plugin will place the ad banner code into the head section of your website and you shall be able to see your desired output.
In case we mis-understood and you were asking about placing an ad onto your site that is below your navigation bar, that is something that would be done via your theme and WP, or using another plugin.
For example, check if your theme has a WIDGET ZONE that is under the navigation bar. If so, you can just add a TEXT widget to that Zone, and place your ad code within it.
The HFCM plugin is moreso for code snippets that are usually added to the <head> or <footer> of a site.
