Dropdown not showing data from BDM

Hello,

I want to create a Dropdown, that Displays all the data from a Business Data ModelTable called "Customer".
I created a custom querry to get all the IDs (called getAllIDs):
SELECT persistenceId
FROM Customer

On my form I created a variable with the following external API:
../API/bdm/businessData/de.test.model.Customer?q=getAllIDs

In the Dropdown I selected then uses this vaiable in the "Available values" field and I enterd PERSISTENCEID as the "Displayed key".

But I'm still not getting any values!?

Thanks!