Here is the solution (see image)

1. Uncheck DNM in 'Option:...' row
2. Enter 1 as default value
3. Save
4. Edit again
5. A new row will appear for 'Existing user ID'
6. Select Source field, i.e uid.
7. Save

This works...

E

Here is the solution (see image)

1. Uncheck DNM in 'Option:...' row
2. Enter 1 as default value
3. Save
4. Edit again
5. A new row will appear for 'Existing user ID'
6. Select Source field, i.e uid.
7. Save

This works...