Command to install Drupal 9 with Composer
create-project can take a version number, and recommended-project has some tags in for 9.x, so for example:

composer create-project drupal/recommended-project mynewproject "9.0-beta1"


Or if you just want the bleeding edge to play with:

composer create-project drupal/recommended-project mynewproject "^9.0" --stability=dev --prefer-dist