wp-user-manager/wpum_render_account_tab_content.php

## wpum_render_account_tab_content.php
<?php

function my_wpum_account_tab_content() {
  // render content
}

add_action( 'wpum_account_page_content_custom-slug', 'my_wpum_account_tab_content' );
	<?php

	function my_wpum_account_tab_content() {
	// render content
	}

	add_action( 'wpum_account_page_content_custom-slug', 'my_wpum_account_tab_content' );