Drop down menu where statement

I would like to make a drop down list which is depends a where statment and a field which i have just field in the form.

Let´s say:

I have a form with field name, city and country. On the drop down list i would like to show all names which are in the country which i selected in the form before. That means someting like on the fly.

i have tried to use in the lookuo wizard where statment "country = '" & values("country") & "'" but this not wokrs.

Somebody have any idear?