I will explain you the logic via data-structure. Assumption: Data model (presenting as if in DB) would be something like
CustomersTable
CustomerID CustomerName CompanyID
CompanyTable
CompanyID CompanyName
Now, to have data for a given customer, a SQL query would be like:
SELECT
CompanyName, CompanyID
FROM
CompanyTable
INNER JOIN
CustomerTable ON CompanyTable.CompanyID = CustomerTable.CompanyID
WHERE
CustomerTable.CustomerID = @customerid
Thus, you get the client associated with the given Customer which you can use and set the company dropdown with.
Try out based on above logic.