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.
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