odoo-survey-addons/survey_input_template_custom/templates/survey_init.xml

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

  <template id="survey_init" inherit_id="survey.survey_init">
    <a role="button" position="replace">
      <a role="button" class="btn btn-primary btn-lg" t-att-href="'/survey/fill/%s/%s' % (slug(survey), token)">
        <t t-if="survey.start_btn_label">
          <span t-field="survey.start_btn_label"/>
        </t>
        <t t-else="">
          Start Survey
        </t>
      </a>
    </a>
  </template>

</odoo>