This web service is a base64 API and is to be used in conjunction with App Inventor for Android.
WP REST API provides OAuth 1.0a and Basic Auth algorithm. Currently it is no way to use OAuth 1.0a on App Inventor, so I use Basic Auth on this app.
Because App Inventor don't have function for base64 encode, so I create a tinywebdb-base64 program, which use App Inventor Tinywebdb feature to get value = base64_encode (tag). App Inventor apps can access this service using the TinyWebDB component and setting the ServiceURL to the URL of this site.
The service returns a value = base64_encode (tag). You can explore how this API works by entering a tag and clicking Get value.