Add New Custom Fields (Meta boxes)

/**
 * Register custom fields.
 */
function my_register_custom_metaboxes() {
    if ( ! class_exists( 'WPEX_Meta_Factory' ) || ! is_admin() ) {
        return;
    }

    new WPEX_Meta_Factory( array(
        'id'       => 'YOUR_UNIQUE_METABOX_ID',
        'title'    => esc_html__( 'Metabox Title', 'text_domain' ),
        'screen'   => array( 'post', 'page', 'portfolio' ), //post types to add the metabox to
        'context'  => 'normal',
        'priority' => 'default',
        // @important since Total Theme Core v1.7.1 you can now pass a function that returns the fields to prevent
        // the fields from being stored in memory and so they are only called when needed.
        'fields'   => array(
			// Select Field
            array(
                'name' => esc_html__( 'Select example', 'text_domain' ),
				'desc' => esc_html__( 'Custom description if needed', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'select',
                'choices' => array(
                    '' => esc_html__( 'Choice 1', 'text_domain' ),
                    'option-2' => esc_html__( 'Choice 2', 'text_domain' ),
                ),
            ),
			// Button Group Field
			array(
                'name' => esc_html__( 'Button Select example', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'button_group',
                'choices' => array(
                    '' => esc_html__( 'Choice 1', 'text_domain' ),
                    'option-2' => esc_html__( 'Choice 2', 'text_domain' ),
                ),
            ),
			// Checkbox Field
			array(
				'name' => esc_html__( 'Checkbox example', 'text_domain' ),
				'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
				'type' => 'checkbox',
			),
			// Text Field
            array(
                'name' => esc_html__( 'Text Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'text',
            ),
			// HTML Field
            array(
                'name' => esc_html__( 'Text Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'text',
				'allow_html' => true,
            ),
			// Textarea Field
            array(
                'name' => esc_html__( 'Textarea Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'textarea',
            ),
			// URL Field
            array(
                'name' => esc_html__( 'URL Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'url',
            ),
			// Date Field
            array(
                'name' => esc_html__( 'Date Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'date',
            ),
			// Upload Field
            array(
                'name' => esc_html__( 'Upload Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'upload',
                'return' => 'id', // this is the value that will be saved, id or url (we recommend always storing attachment ID's)
                'media_type' => 'image', // image, pdf, video, audio - important to define this so it renders nicely
            ),
			// Group Field
            array(
                'name' => esc_html__( 'Group/Repeater Type', 'text_domain' ),
                'id'   => 'YOUR_UNIQUE_CUSTOM_FIELD_NAME',
                'type' => 'group',
				'group_sort' => true, // allow drag/drop sorting of the fields
				'group_title' => esc_html__( 'The Title for each group item (optional)', 'text_domain' ),
                'fields' => array(
                    // Add your repeater fields here
                ),
            ),
        )
    ) );
}
add_action( 'admin_init', 'my_register_custom_metaboxes' );