const donationWallet = '23434';
const amount = 12.4;
const transactionObject = {
  from: web3.eth.accounts[0],
  to: donationWallet,
  value: amount,
}

web3.eth.sendTransaction(transactionObject, function(){
  alert('ok');
})

 <script>
  function donate() {

  var web4 = new Web3(web3.currentProvider);
  const donationWallet = '23434';
  const amount = parseFloat(document.getElementById('amount').value);
  const transactionObject = {
    from: web4.eth.accounts[0],
    to: donationWallet,
    value: amount,
  }

  web4.eth.sendTransaction(transactionObject, function(){
    alert('ok');
  })
  
}
  
</script>


<input type="number" id="amount" min="0.01" max="10" step="0.05">
<button onclick="donate()">Donate</button>