A company want to distribute an iOS app I developed on their behalf. But in order to enrol into the Apple Developer Program, they are required to activate Two-factor authentication.
But they don't have a Mac, or an iOS device to receive an authentication code. How do I enable Two-factor authentication for their Apple ID?
You can use a mobile number to receive verification code by SMS. Refer to this Apple Support document, Two-factor authentication for Apple ID
Trusted phone numbers
A trusted phone number is a number that can be used to receive verification codes by text message or automated phone call. You must verify at least one trusted phone number to enroll in two-factor authentication.
You should also consider verifying an additional phone number you can access, such as a home phone, or a number used by a family member or close friend. You can use this number if you temporarily can't access your primary number or your own devices.
To enable your mobile phone to receive two-step verification code, go to Manage your Apple ID website, login with the Apple ID, go to Security section, click edit and add a trusted phone number.
You'd be asked to verify it. Once you have it verified, you can use it to receive the two-step verification code as an SMS.