How do I get X value into my form?


You need a shortcode that returns that value.

You can use one of the included shortcodes, a shortcode from a third party component like a plugin or theme, or write your own custom shortcode

As long as your shortcode is written properly and it returns a string value that can be displayed in a text input, you can use it.

Remember, CF7 DTX is just the conduit for putting the value returned from your shortcode into the CF7 text input.