odoo-event-addons/event_to_calendar/views/event_event.xml

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

  <record id="view_event_form_inherit_event_to_calendar" model="ir.ui.view">
    <field name="name">event.event.view.form.inherit</field>
    <field name="model">event.event</field>
    <field name="inherit_id" ref="event.view_event_form" />
    <field name="arch" type="xml">
      <xpath expr="//header/*[1]" position="before">
        <button type="object" name="add_to_agenda" string="Add to my agenda" class="oe_highlight"
          icon="fa-calendar-plus-o" attrs="{'invisible':[('in_user_agenda','=',True)]}" />
        <button type="object" name="remove_from_agenda" string="Remove from my agenda" class="btn btn-secondary"
          icon="fa-calendar-minus-o" attrs="{'invisible':[('in_user_agenda','!=',True)]}" />
      </xpath>
      <xpath expr="//div[hasclass('oe_button_box')]" position="inside">
        <button type="object" name="display_meetings" class="oe_stat_button" icon="fa-calendar">
          <field name="meeting_count" widget="statinfo" string="Meetings" />
        </button>
      </xpath>
      <field name="date_tz" position="after">
        <field name="in_user_agenda" invisible="1" />
      </field>
    </field>
  </record>

  <record id="event_add_to_agenda_action_server" model="ir.actions.server">
    <field name="name">Add to agenda</field>
    <field name="model_id" ref="event.model_event_event" />
    <field name="binding_model_id" ref="event.model_event_event" />
    <field name="state">code</field>
    <field name="code">if records:
      records.add_to_agenda()
    </field>
  </record>

  <record id="event_remove_from_agenda_action_server" model="ir.actions.server">
    <field name="name">Remove from agenda</field>
    <field name="model_id" ref="event.model_event_event" />
    <field name="binding_model_id" ref="event.model_event_event" />
    <field name="state">code</field>
    <field name="code">if records:
      records.remove_from_agenda()
    </field>
  </record>

</odoo>