SMS.Studio has recently launched its custom fields via API call.

The end point would be:
https://api.sms.studio/customer/contact

Example for an API call:
https://api.sms.studio/customer/contact?api_token=YOUR_SECRET_TOKEN&response_type=json&list=123?=12123456789&custom[signup_id]=SIGNUP_ID_GOES_HERE

You can create your own custom fields by editing within the brackets next to 'custom'.

For example:
list=LIST_ID?=12123456789&custom[Order_Number]=test123
custom[CustomTracker]=b990234

Etc...