contract/agreement_project/views/agreement_view.xml

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

    <record model="ir.actions.act_window"
            id="action_project_task_agreement_specific">
        <field name="name">Tasks</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">project.task</field>
        <field name="view_type">form</field>
        <field name="view_mode">tree,form</field>
        <field name="domain">[('agreement_id', '=', active_id)]</field>
        <field name="help" type="html">
            <p class="oe_view_nocontent_create">
                Create Tasks
            </p>
        </field>
    </record>

    <record id="partner_agreement_form_view_task" model="ir.ui.view">
        <field name="name">agreement.form.task</field>
        <field name="model">agreement</field>
        <field name="inherit_id" ref="agreement.partner_agreement_form_view"/>
        <field name="arch" type="xml">
            <xpath expr="//div[@name='button_box']" position="inside">
                <button name="%(action_project_task_agreement_specific)d"
                        type="action"
                        class="oe_stat_button" icon="fa-tasks">
                    <div class="o_field_widget o_stat_info">
                        <span class="o_stat_value">
                            <field name="task_count" widget="statinfo"
                                   nolabel="1"/>
                        </span>
                        <span class="o_stat_text">Tasks</span>
                    </div>
                </button>
            </xpath>
        </field>
    </record>

</odoo>