code: public CreateUserPoolRequest withUsernameAttributes(String... usernameAttributes) {
          if (this.usernameAttributes == null) {
              setUsernameAttributes(new java.util.ArrayList<String>(usernameAttributes.length));
          }
          for (String ele : usernameAttributes) {
              this.usernameAttributes.add(ele);
          }
          return this;
      }
docs: <p>
  Specifies whether email addresses or phone numbers can be specified as usernames when a user signs up.
  </p>
  <p>
  <b>NOTE:</b> This method appends the values to the existing list (if any). Use
  {@link #setUsernameAttributes(java.util.Collection)} or {@link #withUsernameAttributes(java.util.Collection)} if
  you want to override the existing values.
  </p>
  
  @param usernameAttributes
  Specifies whether email addresses or phone numbers can be specified as usernames when a user signs up.
  @return Returns a reference to this object so that method calls can be chained together.
  @see UsernameAttributeType