View file application/appearance/standart/elements/modals/mailing.tpl

<div class="modal fade" id="massMailing" tabindex="-1" aria-labelledby="massMailingLabel" aria-hidden="true">
	<div class="modal-dialog">
		<div class="modal-content">
			<div class="modal-header">
				<h1 class="modal-title fs-5" id="massMailingLabel">Массовая рассылка</h1>
				<button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
			</div>
			
			<div class="modal-body">
				<form id="formMassMailing">
					<div class="Recipients mb-2">
						<div class="Header">
							<div class="List"></div>
							
							<button type="button" class="btn btn-primary" onclick="showSearchPeople(2);">
								<i class="bi bi-people-fill"></i>
							</button>
						</div>
					</div>
					
					<textarea name="message" class="form-control mb-2" rows="3" placeholder="Сопроводительное письмо"></textarea>
					<input type="file" class="form-control" name="files[]" multiple accept="{func Uploads:AcceptList()}">
					
					<input type="submit" class="btn btn-primary w-100 mt-4" value="Отправить">
				</form>
			</div>
		</div>
	</div>
</div>

<script>
	$('#formMassMailing').submit(function(Event) {
		Event.preventDefault();
		
		SendPost('/application/backstage/Messages.php', {
			massMailing: 1, this: this
		}, (Result) => {
			$('#formMassMailing').trigger('reset');
			$('#massMailing').modal('hide');
		});
	});

	$('#massMailing').modal('show');
</script>