Can I insert a subscribe form via shortcode?

Sure. Postmatic comes with a shortcode which will allow you to display the subscribe form within a post, page, or otherwise. In order to display the Postmatic subscribe form use the following: 

[postmatic_subscribe_widget]

By default the shortcode will display the subscribe form and ask for both name and email address. If you would prefer to not collect names set collect_name="false" like this:

[postmatic_subscribe_widget collect_name="1"]

You can also display a title with the form:

[postmatic_subscribe_widget title="Subscribe Now!"]