Use GET /custom-fields to retrieve a list of all custom fields.
The response for GET /custom-fields looks like this. Note the internalId at the top:
[
{
"internalId": "3793",
"scriptId": "custentity_wc_picture",
"name": "WC Picture",
"subtab": {
"internalId": "265",
"name": "Asset Info"
},
"type": {
"value": "entityCustomField"
},
"fieldType": "_image"
},
{
"internalId": "3794",
"scriptId": "custentity_wc_related_asset",
"name": "Related Asset",
"subtab": {
"internalId": "265",
"name": "Asset Info"
},
"type": {
"value": "entityCustomField"
},
"fieldType": "_listRecord"
}
]
