How do I get a radio code for my tape deck?

[1993 Silver 190]

I have a Factory Alpine tape deck from a 1987 535is that I am going to put in my 1988 325is. Unfortunately, I don't have the security code. The stereo has two numbers on the top:

PSE0563210
L229B7

I asked the dealership and they said I needed the serial number. How do I get the radio code?

Thanks,

Don