custom-rest-api-endpoint.php

<?php
/**
 * POST to endpoint /wp-json/my-custom-endpoint/v1/save-string
 *
 * Send a string and use "Application Passwords" plugin and save to option
 */

add_action( 'rest_api_init', 'my_custom_endpoint', 10 );

/**
 * Registers a custom REST API endpoint.
 *
 * @return void
 */
function my_custom_endpoint() {

	register_rest_route(
		'my-custom-endpoint/v1',
		'save-string',
		array(
			'methods'  => 'POST',
			'callback' => 'my_custom_endpoint_callback',
		)
	);

}

/**
 * The callback for your custom endpoint.
 *
 * @param WP_REST_Request $request The REST request object.
 * @return void
 */
function my_custom_endpoint_callback( $request ) {

	/**
	 * These are headers that your
	 * microservice's request should contain
	 *
	 * The password should be set using
	 * the "Application Passwords" plugin
	 * https://wordpress.org/plugins/application-passwords/
	 */
	$user_login    = $request->get_header( 'user_login' );
	$user_password = $request->get_header( 'user_password' );

	$user = wp_signon(
		array(
			'user_login'    => $user_login,
			'user_password' => $user_password,
			'remember'      => false,
		)
	);

	if ( is_wp_error( $user ) ) {
		wp_send_json_error( 'User must be authenticated', 401 );
		return;
	}

	// This is where you would do something with the request.
	$params = $request->get_body_params();

	if ( ! isset( $params['string'] ) ) {
		wp_send_json_error( 'Must include string param.', 400 );
		return;
	}

	// Update an option. You can do anything here.
	$string = sanitize_text_field( $params['string'] );
	update_option( 'test-string', $string );

	// Send JSON success back to the microservice.
	wp_send_json_success( 'ok' );

}