How to pass a variable to a modal dialog

I'm using some javascript to get the selected row of an IG. The data I select is an ID that i want to pass to a item on the modul dialog but the typical ways of passing them (f.e. setting the items when defining the link to the dialog) doesn't work.

The process in the end should work like this:

1. The user selects a row on Page1 (Is Working)
2. Selected Data is passed to an hidden item on page1 (Is Working)
3. Click on Button on page1 opens the modul dialog (Is Working)
4. The display only Item "ID" on the dialog page is set to the hidden item from page1 (Not Working)

Thanks a lot in advance!