Add category to submitted events automatically

functions.php

<?php
add_action( 'tribe_community_event_created', 'rt_ce_auto_add_cat_for_submission' );

function rt_ce_auto_add_cat_for_submission( $event_id ) {
	$tribe_ecp  = Tribe__Events__Main::instance();

	// replace with your required category ID.
	$cat_ids = [ 20 ]; // for multiple values add like this [ 20, 21 ]

	wp_add_object_terms( $event_id, $cat_ids, $tribe_ecp->get_event_taxonomy() );
}

@rafsuntaskin what is the purpose of line 6?

Good catch, that one is not needed.