I don't know hoe to add a extra button of the header of the tree view?

<record model="ir.ui.view" id="crm_todo_tree_view_opportunity">
 <field name="name">Todo Opportunities Tree</field>
 <field name="model">crm.lead</field>
 <field name="arch" type="xml">
<tree string="Opportunities" fonts="bold:message_unread==True" colors="gray:probability == 100;red:date_deadline and (date_deadline &lt; current_date)">
 <field name="date_deadline" invisible="1"/>
 <field name="create_date" invisible="1"/>
 <field name="name" string="Opportunity"/>
 <field name="partner_id" string="Customer"/>
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</tree>
</field>
</record>
in this view there is button CREATE, after that i need a new button.
Please help me
thanks in advance.