This is called a conditional value list.
If you have a field in customers that stores the account name. You can define an unstored calculation with a "text" result type that uses:
Get ( AccountName )
and use it as a match field to the text field in your customers list.
For more on conditional value lists, see these links:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.