You should be able to accomplish this easily with server-side scripting. For example, create a custom script named addThing, then invoke the script from your client by calling POST /rest/system/script/addThing.
The script should be written to call platform.api.post("db/my_table_name", item), for example.
Here is the relevant documentation page:
https://github.com/dreamfactorysoftware/dsp-core/wiki/API-Access-via-Scripts
And a quick tutorial: