The user / Apple ID with the role of account holder can request the distribution path via the following link:
https://developer.apple.com/support/unlisted-app-distribution/

Please enter your data.
App name:
Individual app name
Apple ID of App:
Individual App ID (can be found in the App Store)

Under the item Describe in detail the business problem your app solves, you can enter the following:
This is an informal- and communication-app for all users which are in business relationship with the company. That means, it's for partners, employees, contractors but also for end customers which are not part of the company.
The question Why do you prefer to distribute this app unlisted on the App Store instead of privately to specific organizations on Apple Business Manager or Apple School Manager? can be answered as follows:
Many users of this app are not part of the company who use their private iPhones. The users are also often outside the country, so the ABM is not suitable.
For What is the estimated size of your app's user base? please enter the value:
Estimated size X employees

Check the categories as follows:
Who is the audience for your app?
Select all.
In what regions will your app be available?
Check the box Select all.
