Looker Actions - Default value

I’ve been trying to setup a default value for my Looker action. It’s to get a fixed string:

"fields": [
            {
                "name": "comment",
                "label": "Comment",
                "type": "text",
                "required": false,
                "text_value": "default"
            },

Which produces the value as expected:

But I was wondering–is it possible to set the existing value for a given row? (it’d be dynamic rather than fixed)

Existing Looker documentation: 

https://docs.looker.com/reference/field-params/action

https://docs.looker.com/sharing-and-publishing/action-hub