Vendor Bank Details - Bank control Key for Spain

Greetings,

Can anyone tell me how to maintain the bank control key for Spain in the vendor master data? I can see that Spain uses the key to check the combination of bank number and bank account number, and it looks like the validation points to table BNKAIN. However, where do I get the two digit bank control key (from the vendor?), and how do I maintain it in the table (this table is buffered)?

Any advice you have would be greatly appreciated, and I will gladly assign points for helpful responses.

Thanks!