Make sure Composer is installed globally, as explained in the installation chapter of the Composer documentation.
Open a command console, enter your project directory and execute:
$ composer require shanbo/recaptcha
recaptcha:
key: '%env(GOOGLE_RECAPTCHA_KEY)%'
secret: '%env(GOOGLE_RECAPTCHA_SECRET)%'
GOOGLE_RECAPTCHA_KEY=
GOOGLE_RECAPTCHA_SECRET=