Hi,

You can pass default value from main model to one2many model using the context.

See Sample:

<field name="timesheet_ids" context="{'default_project_id': project_id}">
    <tree editable="bottom" string="Timesheet Activities" default_order="date">
        <field name="date"/>
        <field name="user_id" invisible="1"/>
        <field name="employee_id" required="1"/>
        <field name="name"/>
        <field name="unit_amount" string="Duration" widget="float_time"/>
        <field name="project_id" invisible="1"/>
    </tree>
</field>

See: How to Set default value while creating record from one2many field

Thanks