FCPXML anchor setting

Hello all!
We are going to generate by software a timeline with fcpxml format, and we can't find how to set in a text element the "anchor" properties?
Hopefully one of you can help us to find it.
We are going to generate by software a timeline with fcpxml format, and we can't find how to set in a text element the "anchor" properties?
Hopefully one of you can help us to find it.