If you use a custom trigger and a custom action you need to define the fields you need in your action as output fields in the trigger (and as input fields on your action). When you post to the action endpoint (most likely as a result from receiving a webhook) you give those fields in the postbody, like:
//Build the post body
const postBody = {
trigger: {
outputFields: {
boardId: event.boardId,
itemId: event.pulseId,
failedId: null,
},
},
};