All API requests require your API Key to be included in the header, which is detailed in the introduction guide. For brevity's sake, this guide does not include the header information
1) Get a list of linkids for the SIMs you want to add a Data Limit to. The easiest way to do this is downloading a CSV of the devices in your organization, which is described here.
2) Make the following API request:
Note: If you do not want the device(s) to have a data limit, set the limit to -1.