Need a simple but complete example of adding metabox to taxonomy

This will add a field called ‘TERM META TEXT’ to your categories. I did take out the nonce but I really think it should go back in. Also, it’s just better to have some sanitization vs. none. This example includes javascript and CSS hooks which you may or may not need but you can quickly see how all the parts go together.

Enjoy!

// REGISTER TERM META

add_action( 'init', '___register_term_meta_text' );

function ___register_term_meta_text() {

    register_meta( 'term', '__term_meta_text', '___sanitize_term_meta_text' );
}

// SANITIZE DATA

function ___sanitize_term_meta_text ( $value ) {
    return sanitize_text_field ($value);
}

// GETTER (will be sanitized)

function ___get_term_meta_text( $term_id ) {
  $value = get_term_meta( $term_id, '__term_meta_text', true );
  $value = ___sanitize_term_meta_text( $value );
  return $value;
}

// ADD FIELD TO CATEGORY TERM PAGE

add_action( 'category_add_form_fields', '___add_form_field_term_meta_text' );

function ___add_form_field_term_meta_text() { ?>
    <?php wp_nonce_field( basename( __FILE__ ), 'term_meta_text_nonce' ); ?>
    <div class="form-field term-meta-text-wrap">
        <label for="term-meta-text"><?php _e( 'TERM META TEXT', 'text_domain' ); ?></label>
        <input type="text" name="term_meta_text" id="term-meta-text" value="" class="term-meta-text-field" />
    </div>
<?php }


// ADD FIELD TO CATEGORY EDIT PAGE

add_action( 'category_edit_form_fields', '___edit_form_field_term_meta_text' );

function ___edit_form_field_term_meta_text( $term ) {

    $value  = ___get_term_meta_text( $term->term_id );

    if ( ! $value )
        $value = ""; ?>

    <tr class="form-field term-meta-text-wrap">
        <th scope="row"><label for="term-meta-text"><?php _e( 'TERM META TEXT', 'text_domain' ); ?></label></th>
        <td>
            <?php wp_nonce_field( basename( __FILE__ ), 'term_meta_text_nonce' ); ?>
            <input type="text" name="term_meta_text" id="term-meta-text" value="<?php echo esc_attr( $value ); ?>" class="term-meta-text-field"  />
        </td>
    </tr>
<?php }


// SAVE TERM META (on term edit & create)

add_action( 'edit_category',   '___save_term_meta_text' );
add_action( 'create_category', '___save_term_meta_text' );

function ___save_term_meta_text( $term_id ) {

    // verify the nonce --- remove if you don't care
    if ( ! isset( $_POST['term_meta_text_nonce'] ) || ! wp_verify_nonce( $_POST['term_meta_text_nonce'], basename( __FILE__ ) ) )
        return;

    $old_value  = ___get_term_meta_text( $term_id );
    $new_value = isset( $_POST['term_meta_text'] ) ? ___sanitize_term_meta_text ( $_POST['term_meta_text'] ) : '';


    if ( $old_value && '' === $new_value )
        delete_term_meta( $term_id, '__term_meta_text' );

    else if ( $old_value !== $new_value )
        update_term_meta( $term_id, '__term_meta_text', $new_value );
}

// MODIFY COLUMNS (add our meta to the list)

add_filter( 'manage_edit-category_columns', '___edit_term_columns' );

function ___edit_term_columns( $columns ) {

    $columns['__term_meta_text'] = __( 'TERM META TEXT', 'text_domain' );

    return $columns;
}

// RENDER COLUMNS (render the meta data on a column)

add_filter( 'manage_category_custom_column', '___manage_term_custom_column', 10, 3 );

function ___manage_term_custom_column( $out, $column, $term_id ) {

    if ( '__term_meta_text' === $column ) {

        $value  = ___get_term_meta_text( $term_id );

        if ( ! $value )
            $value="";

        $out = sprintf( '<span class="term-meta-text-block" style="" >%s</div>', esc_attr( $value ) );
    }

    return $out;
}

// ADD JAVASCRIPT & STYLES TO COLUMNS

add_action( 'admin_enqueue_scripts', '___admin_enqueue_scripts' );

function ___admin_enqueue_scripts( $hook_suffix ) {

    if ( 'edit-tags.php' !== $hook_suffix || 'category' !== get_current_screen()->taxonomy )
        return;

    // ADD YOUR SUPPORTING CSS / JS FILES HERE
    // wp_enqueue_style( 'wp-color-picker' );
    // wp_enqueue_script( 'wp-color-picker' );

    add_action( 'admin_head',   '___meta_term_text_print_styles' );
    add_action( 'admin_footer', '___meta_term_text_print_scripts' );
}

// PRINT OUR CUSTOM STYLES

function ___meta_term_text_print_styles() { ?>

    <style type="text/css">
        .column-__term_meta_text { background-color:rgb(249, 249, 249); border: 1px solid lightgray;}
        .column-__term_meta_text .term-meta-text-block { display: inline-block; color:darkturquoise; }
    </style>
<?php }

// PRINT OUR CUSTOM SCRIPTS

function ___meta_term_text_print_scripts() { ?>

    <script type="text/javascript">
        jQuery( document ).ready( function( $ ) {
             $input_field = $( '.term-meta-text-field' );
             // console.log($input_field); // your input field
        } );
    </script>
<?php }