how do I set the default value for a field from the web interface? | Odoo

First, you have to activate the 'developer mode'.

1. Click on your name on the upper right corner
2. Click on 'About OpenERP'
3. Click on 'Activate the developer mode'

Now, you should see a selection box on the top of each form (above the edit/create buttons). An entry of this selection box is 'Set Defaults'.

You need to display the form view of a record to see it. The selected record should have the value you want as a default.

In the 'Set Defaults' form, you are able to choose the value from the current form which you want as a default value. You can also choose if it is applicable to all users or only to you.