This code adds a shortcode [um_friends_button] that displays the "Add Friend" button. This code requires the "Ultimate Member" and "Ultimate Member - Friends" plugins to be installed.

um_friends_button_shortcode.php

<?php

/**
 * Add a shortcode [um_friends_button] that displays the "Add Friend" button.
 *
 * Attributes:
 * - (int) post_id - The post/page ID. Optional. Default: current post/page ID.
 * - (int) user_id - The user ID. Optional. Default: requested user ID or the post/page author ID.
 * 
 * Example: [um_friends_button user_id="20"]
 *
 * Notes:
 * - the button is shown for logged in users only.
 * - the button is hidden if the attribute `user_id` is the current user.
 */
if ( defined( 'um_friends_version' ) && ! shortcode_exists( 'um_friends_button' ) ) {

	function um_friends_button_shortcode( $atts = array() ) {
		$def_atts = array(
			'post_id' => get_the_ID(),
			'user_id' => um_get_requested_user(),
		);

		$args = shortcode_atts( $def_atts, $atts, 'um_friends_button' );

		$user_id1	 = is_numeric( $args['user_id'] ) ? $args['user_id'] : ( is_numeric( $args['post_id'] ) ? get_post( $args['post_id'] )->post_author : 0 );
		$user_id2	 = get_current_user_id();

		if ( $user_id1 > 0 && $user_id2 > 0 ) {
			wp_enqueue_script( 'um_friends' );
			wp_enqueue_style( 'um_friends' );

			return '<div class="um um-friends-button-shortcode">' . UM()->Friends_API()->api()->friend_button( $user_id1, $user_id2 ) . '</div>';
		}
	}

	add_shortcode( 'um_friends_button', 'um_friends_button_shortcode' );
}