Custom Field Name for Yoast SEO

Hi, I’m trying to set up Yoast SEO plugin so that in the meta description of my Testimonial entries it will display the testimonial from the page. They do have a function for custom post types in their template variables: https://kb.yoast.com/article/146-yoast-wordpress-seo-titles-metas-template-variables

It says that I should be able to use this Replaced with a posts custom field value in order to do so: %%cf_<custom-field-name>%%

I’ve looked through your code and no matter what I try I cannot seem to get it to work. What are your custom field names? By looking at the code they are testimonial_url, testimonial_company, testimonial_url, testimonial_content but when I use this formula it does not work:
%%cf_<testimonial_content>%%

Any ideas?

https://www.ads-software.com/plugins/testimonials-widget/