Mage2.PRO extensions are declared in the mage2.pro composer-type repository: https://mage2.pro/ssh/packages.json.
You need declare the mage2.pro repository for your Composer.
Open the COMPOSER_HOME/config.json file (or create it if it is absent), and add the link to the https://mage2.pro/ssh repository.
See also: Where is my «COMPOSER_HOME» directory located?
For example, if the COMPOSER_HOME/config.json is empty, then add the following code to it:
{
"repositories": [
{
"type": "composer",
"url": "https://mage2.pro/ssh"
}
]
}