Is there a way to reference "field_key" value for Custom Object associated with ticket?

Hi there! 

First time posting here but long time reader. 

I am looking to reference a “field_key” value with a Custom Object that I associated with a ticket. For context, I have a lookup field in my ticket directly associated with the records in the Custom Object and I would like to pull the value one of the Custom Object fields to use in an API call externally. 

I've tried to use the following placeholder in the JSON of my API call but it is returning a blank value:

{{custom_objects.myobjectkey.custom_fields.thefieldkey}}

I've ensured that there is no typos in my copying of the object key and field key into the placeholder. Not sure if I need to reference the field ID value of the lookup field instead.

Does anyone know if there is a way to reference this data? Or is what I am trying to achieve not possible at this time?