Since you posted what looks like VB.NET code, I take it this is not in a stored procedure?
It's easier to do in a stored procedure, but you need to get the @@IDENTITY of the record that was added in the first INSERT command. You have nothing returning that ID. Once you get the ID, it's trivial to put it in the INSERT of the second table.
] for a discussion of the concept.
Now, other problems I see with your code is that you're using string contatentation to build the SQL queries. DON'T! Use parameterized queries instead, or better yet, a stored procedure to do this, and populate the parameters using SqlParameter objects.
You're also not validating the data you have in the textbox fields.
Want to know why this is bad?? What do you think would happen if I typed:
x','x'); DROP TABLE Clients; --
into either of those textboxes??
Try reading this article
] on SQL Injection Attacks and find out how to avoid this problem.