How to create memory table with btree index?

Can you help me to explain, how to create Memory table with B-Tree Index in MySQL?

You can specify USING clause provided by MySQL at table creation time to ask for B-Tree Index on a particular column. The following example declares a B-Tree index on the username column:

CREATE TABLE users (
        id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
       username CHAR(15) NOT NULL,
 pwd CHAR(15) NOT NULL,
      INDEX USING BTREE (username),
       PRIMARY KEY(id)
) Engine=MEMORY;