How do I set register_meta for a specific CPT?

register_post_meta has a post type parameter, or just set object_subtype to the post type.

register_meta(
    'post',
    '_duib_cl_heading',
    [
        'auth_callback'  => '__return_true',
        'default'        => __( '', '_duib_cl_heading' ),
        'show_in_rest'   => true,
        'single'         => true,
        'type'           => 'string',
        'object_subtype' => 'testimonial',
    ]
);

Or

register_post_meta(
    'testimonial',
    '_duib_cl_heading',
    [
        'auth_callback' => '__return_true',
        'default'       => __( '', '_duib_cl_heading' ),
        'show_in_rest'  => true,
        'single'        => true,
        'type'          => 'string',
    ]
);