I'm using this and works fine.

<filter string="Ecuadorian States" name="ecuadorian_states" domain="[('country_id', '=', %(base.ec)d)]"/>

Thanks for your reply,

Any workaround to hardcode it in the xml?

Hi ,

Go to the module you want to create the filter for . 

Now create a view/ Filter the save it.(You can only create a view with the given fields in filter list.)

Now go to user define filter . you can find the view .if you want you can change it there as well.

Ps - there are some fields you cannot see in the custom filter list initially. You can add the fields using studio.

Hope this helps.

.