We use a unique SSL-certified single-purpose domain for each Direct Route to automate the direct routing setup. 

We create a new Microsoft user in the enterprise called "DirectRouting User" with a UserName' do not delete'. We assign this User an available Microsoft User license, Phone System, or comparable add-on.

The DirectRouting user is an arcane necessity to "anchor" the domain we create for the direct routing setup. If you create a domain, but don't put a user in it, it isn't "real" as far as Microsoft systems are concerned.

The licenses are assigned to the Direct Routing user for the same reason. They must be present when the direct route is created, but can be reclaimed once established.

However, the DirectRouting user must remain (though it does not need any licenses after setup).

Once Direct Routing is set up, the “Direct Routing User” Microsoft licenses can be unassigned and used elsewhere. One Direct Routing User will be created for each Direct Route configured for the enterprise, and each user will consume a Microsoft license during setup. 

Note that the Direct Routing user does not need to log in, and you can block login for this user in the Microsoft Admin Center: