You can get the id of the form containing the button which was clicked by reading the form.id property from the event target:
function sendReminder(e) {
e.preventDefault();
let formId = e.target.form.id;
console.log(formId);
}
<form id="sendReminderForm{{ user.id }}" method="post" action="">
<input type="hidden" name="policyId" value="{{ policy.id }}">
<input type="hidden" name="userId" value="{{ user.id }}">
<button class="btn btn-warning btn-xs" onclick="sendReminder(event)" type="submit">
<span class="fa fa-paper-plane"></span> Remind
</button>
</form>
However it's worth noting that using onclick, and the other onX event attributes, are no longer good practice and should be avoided.
The better method to achieve your goal is to use unobtrusive event handlers like this:
document.querySelectorAll('form').forEach(form => {
form.addEventListener('submit', e => {
e.preventDefault();
let formId = e.target.id;
console.log(formId);
});
});
<form id="sendReminderForm{{ user.id }}" method="post" action="">
<input type="hidden" name="policyId" value="{{ policy.id }}">
<input type="hidden" name="userId" value="{{ user.id }}">
<button class="btn btn-warning btn-xs" type="submit">
<span class="fa fa-paper-plane"></span> Remind
</button>
</form>
Here's the same logic as above in jQuery, as you tagged it in the question:
$('form').on('submit', e => {
e.preventDefault();
console.log(e.target.id);
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form id="sendReminderForm{{ user.id }}" method="post" action="">
<input type="hidden" name="policyId" value="{{ policy.id }}">
<input type="hidden" name="userId" value="{{ user.id }}">
<button class="btn btn-warning btn-xs" type="submit">
<span class="fa fa-paper-plane"></span> Remind
</button>
</form>