You have to use http POST.
The URL starts with the server address and service , followed by /api/x3/erp/folder_name/class, then by ?representation=representation.$create.
The body contains the values of the properties.
Have a look at the following image, it shows an example with POSTMAN on a single YLEAD entity that is present on your SEED folder
The only point you should be aware is that X3 requires uuid values if you embed a collection (one per collection line). But on a simple class, it goes straightforward without any technical field.