Let's set up the "Audience". Go to "Configure" => "Audiences" section. And click "new audience".

Then click "Create a custom audience".

Enter a name for the new audience (for example, "Callback_calls"). After that, set up the conditions:

1. Click "Events" and select Ringostat_calls.

2. Add the "pool_name" parameter.

3. For conditions, select "exactly matches (=)" and set the value: "callback".

4. Don't forget to specify "Audience Trigger" without spaces. For example, "Callback_calls".

The last step is to set up conversion for this audience. Go to "Configure" => "Conversions". And click "New conversion event".

Give the conversion name the same as the audience trigger — "Callback" and save.

Conversion events are automatically enabled.