odoo-crm-addons/crm_lead_assign_me/views/crm_lead.xml

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

  <record id="view_crm_lead_kanban_view_inherit" model="ir.ui.view">
    <field name="name">crm.lead.view.kanban.inherit</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.view_crm_lead_kanban" />
    <field name="arch" type="xml">
      <xpath expr="//div[hasclass('oe_kanban_bottom_right')]" position="inside">
        <a role="button" type="object" name="action_assign_to_me"
          class="oe_link" attrs="{'invisible': [('user_id', '!=', False)]}">
          <i class="fa fa-user-plus" role="img" title="Assign to me" aria-label="Assignation button" />
        </a>
      </xpath>
    </field>
  </record>

  <record id="crm_case_kanban_view_leads_view_inherit" model="ir.ui.view">
    <field name="name">crm.lead.view.kanban.inherit</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_kanban_view_leads" />
    <field name="arch" type="xml">
      <xpath expr="//div[hasclass('oe_kanban_bottom_right')]" position="inside">
        <a role="button" type="object" name="action_assign_to_me"
          class="oe_link" attrs="{'invisible': [('user_id', '!=', False)]}">
          <i class="fa fa-user-plus" role="img" title="Assign to me" aria-label="Assignation button" />
        </a>
      </xpath>
    </field>
  </record>

  <record id="crm_case_tree_view_leads_inherit_crm" model="ir.ui.view">
    <field name="name">crm.lead.view.tree.inherit</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_tree_view_leads" />
    <field name="arch" type="xml">
      <field name="user_id" position="before">
        <button type="object" name="action_assign_to_me"
          class="oe_link" icon="fa-user-plus"
          title="Assign to me" aria-label="Assignation button"
          attrs="{'invisible': [('user_id', '!=', False)]}" />
      </field>
    </field>
  </record>

  <record id="crm_case_tree_view_oppor_inherit_crm" model="ir.ui.view">
    <field name="name">crm.lead.view.tree.inherit</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_tree_view_oppor" />
    <field name="arch" type="xml">
      <field name="user_id" position="before">
        <button type="object" name="action_assign_to_me"
          class="oe_link" icon="fa-user-plus"
          title="Assign to me" aria-label="Assignation button"
          attrs="{'invisible': [('user_id', '!=', False)]}" />
      </field>
    </field>
  </record>

  <record id="crm_lead_view_form_inherit_crm" model="ir.ui.view">
    <field name="name">crm.lead.view.form.inherit</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_lead_view_form" />
    <field name="priority">1000</field>
    <field name="arch" type="xml">
      <xpath expr="//button[@name='action_set_won_rainbowman']" position="before">
        <button type="object" name="action_assign_to_me" string="Assign to me"
          class="oe_highlight" icon="fa-user-plus" data-hotkey="m"
          attrs="{'invisible': [('user_id', '!=', False)]}" />
      </xpath>
    </field>
  </record>

</odoo>