It's not really clear from the documentation, but there are two ways to translate models to OBJ:
1) Direct translation
Some file formats support translation of the file to OBJ directly, i.e. without translating the file to SVF first. You can find the list of such file formats on the Supported Translations page of the documentation, they will have OBJ listed - see picture on top
If you try to translate a file that does not support this, then you'll get a 400 / Bad Request when using POST job with the following message in the x-ads-troubleshooting header :
The system does not support this file format.
2) Indirect translation
In this case you have to translate the file to SVF first.
All formats support this way of getting an OBJ and it's considered geometry extraction which requires extra parameters: modelGuid and objectIds. In the documentation these parameters have the note "Required for geometry extraction."
There is a tutorial here that shows this two step process: