Hi,

If you like to set a default value in XML, for a particular page or view, then you can consider the following option: In the "action" record that opens your Form View, you can add an entry in a "context" dictionary, like this:

<field name="context">{'default_user_id': uid}</field>

OR

use default=lambda self: self.env.user in the field

OR

In defaults, you can only set a default value for the field and not the domain. Instead, u can do the other way, add another field which will fetch the domain value and that field tries to use it as a domain to your fields.

current_user_id = fields.Many2one('res.users', default=lambda self: self.env.user)

user_id = fields.Many2one('res.users', domain="[('user_id', '=', current_user_id)]")

Regards