Which endpoint should I use to search a company or contact by a custom attribute in the SendPulse CRM API?

Hi everyone,
I’m integrating with the SendPulse CRM API and I need to search for a company or a contact by a custom attribute (for example, “CUIT”, “NIT”, or any other field I’ve created).

Here’s what I’ve tried so far:

• Created the custom attribute in the company module.

• Tested several endpoints listed in the SendPulse CRM API documentation: https://sendpulse.com/integrations/api/crm

• Sent a request like this to filter by that attribute:

{
  "attributes": [
    {
      "id": 969197,
      "expression": "=9011174417"
    }
  ]
}

However, I keep getting this error:
422 - The selected attributes.0.expression is invalid.

I also tried variations such as removing the “=”, using “like”, or switching the endpoint (/search, /filter), but none of them worked.

My questions for the community are:

1. What is the correct endpoint (URL and HTTP method) to search companies or contacts by a custom attribute?

2. What is the correct JSON body format for this type of filter? (For example, should I use expression, value, or another key?)

3. Are there any limitations or conditions on custom attributes (e.g. text, number, list) that prevent this type of search?

4. Does anyone have a working example of a successful API request that filters records by a custom attribute in SendPulse CRM?

Any help, examples, or insights would be greatly appreciated.
Thanks in advance!