hi 
you need to inherit this view  project.view_project_task_pivot and add your field :

an example:

    <record id="view_project_task_pivot_inherited" model="ir.ui.view">

            <field name="name">project.task.pivot.inherited</field>

          <field name="inherit_id" ref="project.view_project_task_pivot"/>

            <field name="model">project.task</field>

            <field name="arch" type="xml">

                    <xpath expr="//field[@ name='project_id' position="after"/>

                    <field name="YOUR FIELD" type="row" />

            </xpath>

       </field>

    </record>