web/web_send_message_popup/static/src/xml/mail.xml

<?xml version="1.0" encoding="UTF-8"?>
<template>
  <t t-extend="mail.compose_message">
    <t t-jquery="a.oe_compose_post:eq(0)" t-operation="replace">
      <a class="oe_compose_post oe_full" t-if="widget.options.compose_placeholder"><t t-raw="widget.options.compose_placeholder"/></a>
    </t>
    <t t-jquery="a.oe_compose_post:eq(1)" t-operation="replace">
      <a class="oe_compose_post oe_full" t-if="!widget.options.compose_placeholder and !widget.options.view_mailbox">Send a message</a>
    </t>
  </t>
</template>