Populate Validation list with Collection

Hello

I have a worksheet with site and customer data (among other things) and would like to restrict what sites a user can select when entering data based on the customer they've selected.

For example, the worksheet has something like the following:
Site Customer
1 Alpha
2 Alpha
3 Alpha
4 Alpha
5 Beta
1 Beta
2 Beta
3 Beta
6 Alpha
4 Beta
1 Gamma
2 Gamma

I thought using a Validation List that populates automatically when the user selects a cell would help. So, if the user selects Alpha as the customer only 1,2,3,4,6 show up in the list. The information in the worksheet can change quite frequently.

I know that I could populate a Collection and then write it to a worksheet that the Validation List could refer to is a possible way to complete this. However, I am wondering if anyone knows of a way that I could simply populate the Validation List with the elements in the Collection?

Thanks for your help in advance.

CKD