Kind of late to the party here but I needed to do this today:
ADO pipeline makes it almost too easy. Treat this just like a powershell or AZcli script. Create a pipeline var & call it with $(var)
IF EXISTS (SELECT * FROM sys.database_principals WHERE name = '$(client)_user')
BEGIN
Print '$(client)_user already exists. Dropping to reinitialize'
DROP USER [$(client)_user]
END