How to Place Ad Code into Header Source Code?

not sure what theme you are using but might be helpful to add a header widget for easy managing/changing later
article that might help
https://wpshout.com/need-know-making-widget-areas-wordpress-themes/

be sure you are using a child theme for all changes to templates (ie header.php etc.)

without a link to your website, it’s difficult to determine a suitable spot for an ad or where to put the code.

Oh, okay; no problem there: https://www.tvruckus.com

Well, according to what I see, our Forefront theme has a child theme for template changes; it is called GeneratePress. That is where I found the header.php.

The ad tag is actually for a pop-up; the code we are being provided with can be placed either in the footer or header.

Or, actually, I may have that backwards; Forefront is a child theme of GeneratePress. This is what it says:

Forefront is a child theme of GeneratePress

It’s not recommended to add CSS to this file, as it will be lost if you ever update this child theme. Instead, use a Custom CSS plugin.

Where do I get a plugin like that?

if you need a plugin to edit CSS:
you can install jetpack plugin module “CUSTOM CSS”

CSS editors are not used for modifications to template files header.php files

I thank you for your patience, as I am going thru this process for the first time, and have no idea what I am doing.

So, are you saying then that CSS editing is a separate issue from the ad tag that I am trying to add, and does not apply to what I am doing? If so, I’ll just forget about that completely, as I don’t want to make any actual changes to the theme or look of it; I just want to add this ad tag.