Below is a basic example that will add a select box right before the submit button.

add_action( 'post_submitbox_misc_actions', 'wpdocs_post_submitbox_misc_actions' );

function wpdocs_post_submitbox_misc_actions( $post ) {
?>

<div class="misc-pub-section my-options">
	<label for="my_custom_post_action">My Option</label><br/>
	<select id="my_custom_post_action" name="my_custom_post_action">
		<option value="1">First Option goes here</option>
		<option value="2">Second Option goes here</option>
	</select>
</div>

<?php
}