In VSCode, both Composer 1 and Composer 2 are currently available. Composer 2 can be run as composer in the terminal, whereas Composer 2 is available as composer1.
Run these two commands
composer config minimum-stability alpha
composer config prefer-stable true
The first one allows installing alpha packages. Available options (in order of stability) are dev, alpha, beta, RC, and stable.
The second one instructs Composer to prefer stable packages when they are available.
More details here: https://getcomposer.org/doc/04-schema.md#minimum-stability
Set the shell environment variable COMPOSER_MEMORY_LIMIT to -1:
COMPOSER_MEMORY_LIMIT=-1 composer install <PACKAGE>