Is it possible to do STS (speech-to-speech) via API?

Is there STS using SSML tag?

If a voice supports STS then you can follow these docs to use it via API:
You can use Speech Synthesis Markup Language (your SSML) as input to control how Resemble generates speech. Resemble automatically handles normal punctation, such as pausing after a period, or speaking a sentence that ends with a question mark as a question. However, in some cases, you may want additional control of Resemble’s synthetic speech. This may include, for example, having certain words pronounced in a specific way, saying a word or sentence with excitement, spelling certain words character by character, and much more.