In the Advanced Settings section, add another sub-section between the current two (Operation/Columns and Retry Logic), titled Headers
The UI for adding headers can be similar to the one for adding columns when creating a table:
Add auto-complete/suggest for the values of a "from env variables" type. Are there any downsides to doing this?

Also, adding some minor edits to the current UI:

Add a ? icon next to the Advanced Settings button with the following tooltip: retry logic and custom headers
~~Remove Advanced - from Advanced - Operation/Columns in advanced settings~~
The Operation/Columns section is shown only if the source table is selected. Let's make it visible with this text in the following line: Select the source schema and table to customize the webhook payload. (instead of the options).
~~In the Operation/Columns section, add an indent to list of columns after each operation.~~
The part under the left nav bar is off when you expand the Advanced Settings section.

@rikinsk Please to review this.

EDIT: Removed references to payload selection config

set event trigger webhook headers from console #523

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