I will explain you the logic via data-structure. Assumption: Data model (presenting as if in DB) would be something like
CustomerID CustomerName CompanyID
Now, to have data for a given customer, a SQL query would be like:
CustomerTable ON CompanyTable.CompanyID = CustomerTable.CompanyID
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.