We use a dependent drop-down ticket field titled "Product" that opens up a second level of "Module".
I have been able to reference both Product and Module before as placeholders in various rules by using the "Insert Placeholder" function and going to Ticket Fields and selecting from a dropdown as shown:
This shows up then as "{{ticket.product}}" and "{{ticket.module}}".
But when using the "Trigger Webhook" under "perform these actions" the "Product {{ticket.product}}" can be found but there is no "Module {{ticket.module}}" for the dependent field.