Forms work in same fashion as anchors or vice versa.
When a user submits a form, say when user make a POST request to siteb.com by submitting the form. The response loads in new context, the orginating site is replaced by the site where the action is defined.
<form action="http://store.sitea.com" class="form-signin">
<h1 style="text-align: center;" class="h2 mb-6 font-weight-normal">Login to buy Gifts</h1><br>
<label for="inputEmail" class="sr-only">Email address</label>
<input type="email" id="inputEmail" class="form-control" placeholder="Email address" required autofocus>
<label for="inputPassword" class="sr-only">Password</label>
<input type="password" id="inputPassword" class="form-control" placeholder="Password" required>
<div class="checkbox mb-3">
<label>
<input type="checkbox" value="remember-me"> Remember me
</label>
</div>
<button class="btn btn-lg btn-primary btn-block" type="submit">Sign in</button>
</form>Conclusion - Any website can make a POST request, but can't read the response in same context.