odoo16/addons/event_crm/views/crm_lead_views.xml

<?xml version="1.0"?>
<odoo>
    <record id="event_registration_action_from_lead" model="ir.actions.act_window">
        <field name="name">Event registrations</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">event.registration</field>
        <field name="view_mode">tree,kanban,form,calendar,graph</field>
        <field name="domain">[('lead_ids', '=', active_id)]</field>
        <field name="context">{'create': False}</field>
        <field name="help" type="html">
            <p class="o_view_nocontent_empty_folder">
                No registration found
            </p>
        </field>
    </record>

    <record id="crm_lead_view_form" model="ir.ui.view">
        <field name="name">crm.lead.view.form.inherit.event.crm</field>
        <field name="model">crm.lead</field>
        <field name="inherit_id" ref="crm.crm_lead_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//button[@name='action_schedule_meeting']" position="after">
                <field name="registration_ids" invisible="1"/>
                <button name="%(event_registration_action_from_lead)d" type="action" class="oe_stat_button" icon="fa-ticket"
                    attrs="{'invisible': [('registration_count', '=', 0)]}" groups="event.group_event_user">
                    <div class="o_stat_info">
                        <field name="registration_count"/>
                        <span class="o_stat_text"> Attendees</span>
                    </div>
                </button>
            </xpath>
        </field>
    </record>

    <record id="crm_lead_action_from_registration" model="ir.actions.act_window">
        <field name="name">Leads</field>
        <field name="res_model">crm.lead</field>
        <field name="view_mode">tree,kanban,graph,pivot,calendar,form,activity</field>
        <field name="domain">[('registration_ids', 'in', active_id)]</field>
        <field name="context">{'create': False}</field>
        <field name="help" type="html">
            <p class="o_view_nocontent_empty_folder">
                No leads found
            </p>
        </field>
    </record>

    <record id="crm_lead_action_from_event" model="ir.actions.act_window">
        <field name="name">Leads</field>
        <field name="res_model">crm.lead</field>
        <field name="view_mode">tree,kanban,graph,pivot,calendar,form,activity</field>
        <field name="domain">[('event_id', '=', active_id)]</field>
        <field name="context">{'create': False}</field>
        <field name="help" type="html">
            <p class="o_view_nocontent_empty_folder">
                No leads found
            </p>
        </field>
    </record>
</odoo>