How do you add additional custom meta fields

If you want to add your own custom post meta field, you need to use filters: testimonials_widget_columns, testimonials_widget_meta_box,testimonials_widget_posts_custom_column, testimonials_widget_data, and testimonials_widget_get_testimonial_html.

These filters are described in the API with links to examples.

Add Options?

See API for filters testimonials_widget_settings and testimonials_widget_validate_settings if you want to add options like hide_linkedin.

Want to Add Testimonials Form Fields?

See Gist Add a field to Testimonials Premium.