This code snippet adds a few custom social fields. You can use this example to add more custom social fields.

um_custom_social_fields.php

<?php 

/**
 * Add more social fields.
 * Add this code to the file functions.php in the active theme directory.
 */
add_filter( 'um_predefined_fields_hook', function( $predefined_fields ) {

	$predefined_fields['pinterest'] = array(
		'title'      => __( 'Pinterest', 'ultimate-member' ),
		'metakey'    => 'pinterest',
		'type'       => 'url',
		'label'      => __( 'Pinterest', 'ultimate-member' ),
		'required'   => 0,
		'public'     => 1,
		'editable'   => 1,
		'url_target' => '_blank',
		'url_rel'    => 'nofollow',
		'icon'       => 'um-icon-social-pinterest',
		'validate'   => 'pinterest_url',
		'url_text'   => 'Pinterest',
		'advanced'   => 'social',
		'color'      => '#e60023',
		'match'      => 'https://www.pinterest.com/',
	);

	$predefined_fields['twitch'] = array(
		'title'      => __( 'Twitch', 'ultimate-member' ),
		'metakey'    => 'twitch',
		'type'       => 'url',
		'label'      => __( 'Twitch', 'ultimate-member' ),
		'required'   => 0,
		'public'     => 1,
		'editable'   => 1,
		'url_target' => '_blank',
		'url_rel'    => 'nofollow',
		'icon'       => 'um-icon-social-twitch',
		'validate'   => 'twitch_url',
		'url_text'   => 'Twitch',
		'advanced'   => 'social',
		'color'      => '#6441a5',
		'match'      => 'https://www.twitch.tv/',
	);

	return $predefined_fields;
} );


add_filter( 'um_admin_field_validation_hook', function( $array ) {
	$array['pinterest_url'] = __('Pinterest URL','ultimate-member');
	$array['twitch_url']    = __('Twitch URL','ultimate-member');
	return $array;
} );

This gist is a part of the article Сustom social fields

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/