This example shows how to add custom predefined field that extends the 'checkbox' field type in Ultimate Member

Custom field that extends the 'checkbox' field type.php

<?php

/**
 * This example shows how to add custom predefined field that extends the 'checkbox' field type in Ultimate Member
 * You can add custom code to the end of the file functions.php in the active theme directory.
 * Important! You have to change variables: $type, $title, $options
 *
 * @author  Ultimate Member support <support@ultimatemember.com>
 * @package Ultimate Member
 * @since   2020-06-25
 * @see     #40961
 */
if ( function_exists( 'UM' ) ) {

	/* YOU SHOULD SET FIELD TYPE, TITLE AND OPTIONS HERE */
	$type = $metakey = 'um_business_type';
	$title = __( 'Business type', 'ultimate-member' );
	$options = array(
			'business type 01',
			'business type 02',
			'business type 03'
	);

	/* add new predefined field */
	add_filter( 'um_predefined_fields_hook', function( $predefined_fields ) use($metakey, $options, $title, $type) {
		$predefined_fields[$type] = array(
				'type' => 'checkbox',
				'title' => $title,
				'label' => $title,
				'metakey' => $metakey,
				'public' => 1,
				'required' => 0,
				'editable' => 1,
				'options' => $options
		);

		return $predefined_fields;
	}, 20 );

	/* set default options */
	$saved_fields = get_option( 'um_fields' );
	if ( isset( $saved_fields[$metakey] ) && ( empty( $saved_fields[$metakey]['options'] ) || $saved_fields[$metakey]['options'] !== $options ) ) {
		$saved_fields[$metakey]['options'] = $options;
		update_option( 'um_fields', $saved_fields );
	}

	/* override options on view and edit */
	add_filter( "um_get_field__$metakey", function( $array ) use( $options ) {
		$array['options'] = $options;
		return $array;
	}, 20 );

	/* override options on save */
	add_filter( 'um_user_edit_profile_fields', function( $fields ) use( $metakey, $options ) {
		if ( isset( $fields[$metakey] ) ) {
			$fields[$metakey]['options'] = $options;
		}
		return $fields;
	}, 20 );

	/* use this field as a member directory filter */
	if ( isset( UM()->member_directory()->filter_fields ) ) {
		UM()->member_directory()->filter_fields[$metakey] = $title;
	}
}

Hi, I have a similar question:
How to overwrite predefined countries? I tried a few times, but couldn't figure it out. Can you help me?

Similar Problem. but for Images. How do you extend the image field to take multiple images as input?