Sending data from Meta Box to email "To" field

Firstly whilst I hope you can help on this forum I’m happy to pay to get this fixed if outside the scope of the forum support.

I’m trying to get an email address which is set in a Portfolio item meta box to be used in the CF7 Mail (2) “To” field. But not working. Outline description below:

At: http://extperts.com/projects/ceo-cfo/
Trying to setup form so that the “To” email address in Mail (2) is dynamically populated from a value in the Portfolio item’s meta box (the developer wasn’t keen to tell me how to implement custom fields on Portfolio items, preferring Meta Boxes). So within metabox.php have included this code:

array(
				'name'    	=> __( 'Extpert_email', EF_TDM ),
				'id'		=> "{$prefix}extpert_email",
				'type'		=> 'text',
			),

I’m using Contact Form 7 – Dynamic Text Extension plugin and have included in the form:

[dynamichidden dynamichidden-810 “CF7_get_custom_field key=’extpert_email'”]

and in Mail (2) “To” field

[dynamichidden-810]

But getting “This field has syntax errors.” and the Mail (2) is not being delivered.

Happy to share screenshots if helpful.

https://wordpress.org/plugins/contact-form-7-dynamic-text-extension/