How to add a default value to an already existing column?

I have an existing column in my SQL Server database. I have tried about everything I can think of but can not get a default value to be added to the column. What works in every other database is

alter table mytable 
  alter column mycolumn set default(now()) --mycolumn is a datetime

How do I do this in SQL Server?

The error I get for that exact syntax is incorrect syntax near the keyword 'set'