HOW TO: What's the best way to load a static lookup with a lot of items?

You'll need to format your lookup entries in JSON format. If you have it in CSV format, you can convert it to JSON using a text editor. Once you have it in JSON format, you can copy and paste it in the API request data. You should load at least one entry via the UI, so that when you look at the data via API, you can tell exactly where the remaining data needs to go.

Example:

"lookups": [
 {
  "name": "5f40018e0b424fea88c2ea234903fef8",
  "map": {
    "DebitLine_": "10100",
    "Order_ItemPrice": "10200",
    "Order_ItemWithheldTax",
    ...
  },
  "allowFailures": false
 }
],