How do I set the DEFAULT value for a non-nullable column to the TOP 1 result of a SELECT query?

I'm trying to add a non-nullable column to an already existing table, and I read somewhere that I need a DEFAULT value in order to do that.
The value for the DEFAULT needs to be equal to the TOP 1 user_id result of the SELECT query shown below. I tried using a variable, but this didn't work.

This is what I tried:

DECLARE @latestuserid AS int
SELECT TOP 1 @latestuserid = user_id from Employees ORDER BY user_id DESC
ALTER TABLE Control ADD nextuserid INT NOT NULL DEFAULT (@latestuserid)

However I get the error:

Variables are not allowed in the ALTER TABLE statement.

If I can't use variables, how do I do this if I don't always know the exact value of that SELECT statement?

You can first define the desired column as nullable and then execute an update command for the new column. After that, you can make it not nullable.