settings.php

<?php
    if ( $hassiteconfig ){

    // Create the new settings page
    // - in a local plugin this is not defined as standard, so normal $settings->methods will throw an error as
    // $settings will be NULL
    $settings = new admin_settingpage( 'local_yourplugin', 'Your Settings Page Title' );

    // Create
    $ADMIN->add( 'localplugins', $settings );

    // Add a setting field to the settings for this page
    $settings->add( new admin_setting_configtext(

        // This is the reference you will use to your configuration
        'local_yourplugin/apikey',

        // This is the friendly title for the config, which will be displayed
        'External API: Key',

        // This is helper text for this config field
        'This is the key used to access the External API',

        // This is the default value
        'No Key Defined',

        // This is the type of Parameter this config is
        PARAM_TEXT

    ) );

}
?>