the custom code:

<form action="action_page.php" method="post">
  <div class="form-group">
    <label for="email">Email address:</label>
    <input type="email" class="form-control" placeholder="Enter email" id="email">
	  <label for="name">ονοματεπωνυμο:</label>
	  <input type="text" class="form-control" placeholder="ονοματεπωνυμο:" id="name">
  </div>
  <div class="form-group">
    <label style="color:red" for="pwd">Password:</label>
    <input type="password" class="form-control" placeholder="Enter password" id="pwd" required="fill this field please" oninvalid="password">
  </div>
  <div class="form-check form-group">
    <label class="form-check-label">
      <input class="form-check-input" type="checkbox"> Remember me
    </label>
  </div>
  <button type="submit" class="btn btn-primary">Submit</button>
<input type="hidden" value="" name="recaptchaResponse"></form>


the bootstrap 4 code in html head section:

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js"></script>
</head>

and in the "action_page.php" must have something like that:

<?php

 $var=$_POST[var];


takinng from elements with $_POST they will colect the data from the form

and then at then end

mail(to,subject,body,header);

the header must include "from: email@domain.com for example"

without the header havining inside in the header the "from:email@domain.com" it will fail to send

?>