oca-mrp/odoo-bringout-oca-event-event_project/event_project/views/project_task_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <!-- Allows to search by event and group by event -->
    <record id="view_task_search_form" model="ir.ui.view">
        <field name="name">Filter and group tasks by event</field>
        <field name="model">project.task</field>
        <field name="inherit_id" ref="project.view_task_search_form" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='partner_id']" position="after">
                <field name="event_id" />
            </xpath>
            <xpath expr="//group" position="inside">
                <filter
                    string="Event"
                    name="event"
                    context="{'group_by': 'event_id'}"
                />
            </xpath>
        </field>
    </record>

    <!-- Add related event to task form view -->
    <record id="view_task_form2" model="ir.ui.view">
        <field name="name">Add related event to task</field>
        <field name="model">project.task</field>
        <field name="inherit_id" ref="project.view_task_form2" />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='partner_id']" position="before">
                <field
                    name="event_id"
                    attrs="{'invisible': [('event_id', '=', False)]}"
                />
            </xpath>
        </field>
    </record>
</odoo>