Good morning,
With the latest i-MSCP stable release, PHP 7.3 isn't available without the PhpSwitcher plugin. There if no way to set the PHP 7.3 version the default PHP version when creating a new customer account or domain/subdomain through an existent account. However, once the PHP 7.3 version, either the packaged, or compiled version, is registered in the PhpSwitcher plugin, your customers can select it through their PhpSwitcher interface.
With the next i-MSCP version, which isn't ready for production yet, PHP 7.3 will be the default PHP version. There is also a feature request opened on the PhpSwitcher project (private repository) to let the sysadmin choose which PHP version must be the default one.