social/mail_forward/views/compose_message.xml

<?xml version="1.0" encoding="utf-8"?>

<!-- © 2014-2015 Grupo ESOC <www.grupoesoc.es>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->

<odoo>

    <record id="compose_form" model="ir.ui.view">
        <field name="name">Forward Email Form</field>
        <field name="model">mail_forward.compose_message</field>
        <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
        <field name="arch" type="xml">
            <data>
                <!-- It's required to fill either `partner_ids` or
                     `destination_object_id` -->
                <xpath expr="//field[@name='partner_ids']" position="attributes">
                    <attribute name="attrs">{
                        'required': [('destination_object_id', '=', False)]
                        }
                    </attribute>
                </xpath>

                <xpath expr="//label[@for='partner_ids']" position="before">
                    <!-- Object where the message will be attached -->
                    <field
                            attrs="{
                        'required': [('partner_ids', '=', [[6, False, []]])]}"
                            name="destination_object_id"/>

                    <!-- If the message has attachments, should we move them to the
                         new object? -->
                    <field
                            name="move_attachments"
                            attrs="{
                        'invisible': [
                            '|',
                            ('destination_object_id', '=', False),
                            ('attachment_ids', '=', [[6, False, []]])]}"/>
                </xpath>
            </data>
        </field>
    </record>

    <record id="compose_action" model="ir.actions.act_window">
        <field name="name">Forward Email</field>
        <field name="res_model">mail_forward.compose_message</field>
        <field name="src_model">mail.message</field>
        <field name="type">ir.actions.act_window</field>
        <field name="view_id" ref="compose_form"/>
        <field name="view_type">form</field>
        <field name="view_mode">form</field>
        <field name="target">new</field>
    </record>

</odoo>