The Field ID (or "Prop ID") is a unique, system-generated identifier for every field in a table. This value is not displayed within the application user interface anywhere, so to get the Field ID for a field, you must retrieve it by means of the Table API endpoint.
To retrieve the Field ID for a field:
Submit a request to the Table API endpoint. The simplest method is to use the version of the Table endpoint that allows you to retrieve table information based on the Table Name (see How to Find the Table Name if you need help finding the Table Name).
Within the API response message, the system lists every field in this table. As part of that field definition, the response includes the Field ID (referred to as the "propId").
Sample Response:
{
"viewId": 1002,
"entityId": 100,
"displayName": "create_date",
"propId": 1030,
"columnName": "create_date"
},