Using WebAPI to track html download in email

Requirement is:

WebAPI URL is embedded in email.

When email is opened and html content/image is downloaded, that has to be tracked.

Tried the following:

WebAPI of method POST. But, this WebAPI URL gives error 404 when tried in browser (as it is post method) and does not download content in email.

  processModel: ProcessModel,
  onSuccess: a!httpResponse(
    statusCode: 200

WebAPI of method GET does not allow to to use a!startProcess() to track the content download.

How to meet this requirement? Thank you for your time reviewing this.

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