Question about modular template (begginer)

I'm new to Grav and started to mess with modular page. I get the idea, and followed the basic tutorial and such. I'm using the "agency" theme which have a modular template named "services". I created a "_services" modular item, but can't figure how to add a service.
The important line in services.html.twig is this one:
{% for service in page.header.services %}

I know my question is pretty basic but how do i add a service item in page.header.services?

The best thing to discover a theme is to check out its skeleton, if there is one. It's basically a demo site you can install. Agency has one, check https://getgrav.org/downloads/skeletons.

It has a services.md page (https://raw.githubusercontent.com/getgrav/grav-skeleton-agency-site/develop/pages/01.home/_services/services.md) that will use that Twig template.