Get field populate in custom table

Hi All,

I have a custom table which consists of purchase group and amount. For example, A $100.

I like to create another field for purchse group name, so it will be "A Mary Doe $100".

The thing is when one do a pull-down of purchase group, one can see the same in this table of purchase group.

However, since this is a custom table, is there a way to populate purchase group name once i enter A in the purchase group ?

thanks

Joyce