<input type="text" id="testo">
<button type="submit" onclick="send()">Invia</button>
<script type="text/javascript" src="send.js"></script>

function send(){
const valore = document.getElementById("testo").value
post('/send', {valore})
window.open("/url", "_self", false)
}

function post (path, data) {
    return window.fetch(path, {
      method: 'POST',
      headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json'
      },
      body: JSON.stringify(data)
    })
  }

app.get('/', (req,res)=>{
res.render('index.ejs')
})

app.post('/invio', (req,res)=>{
    valore = req.body.valore
    app.get('/url', (req,res)=>{
        res.render('testo.ejs', {valore: valore})
    })
})

<%= valore %>