Add multiple mirrored items using Integromat/

I have a board containing high-level goals, each of which may be linked to multiple tasks in another board. A mirror column contains the links to each task. Each task may only be linked to one goal, but each goal may have many linked tasks.

I am using to trigger when the tasks board is updated. A new task, linked to a goal, triggers to update the goal’s mirror column with a link to the task. However, when I map the ID of the new task to the goal’s mirror column, it overwrites the current content - therefore we end up with only one task (the most recently added one). I have tried adding ‘Parsed Value.Linked Pulse IDs:Linked Pulse IDs’ to the value to enter into the mirror column, but this has not worked.

Is there a way to add items to a mirror column one at a time, whilst preserving the items already in that column?

Ok, I have been working on this a little, and I think I understand that I should use the GraphQL module to send a mutation. I have written the following:

item_id: {{1.mappable_column_values.connect_boards.linkedPulseIds[].linkedPulseId}}, 
column_id: "connect_boards4", 
board_id: 2747663520, 
value:{{if(6.parsed_value.linkedPulseIds[].linkedPulseId + "="""""; "{""item_ids"":[" + + "]}"; "{""item_ids"":[" + 6.parsed_value.linkedPulseIds[].linkedPulseId + "," + + "]}")}}, 
create_labels_if_missing: true
) {

However on run I get

The operation failed with an error. [200] [{"message":"Parse error on \"item_ids\" (STRING) at [6, 8]","locations":[{"line":6,"column":8}]}]

I assume that there is something wrong with the JSON formatting of ‘item_ids’, but what I have there matches the examples in documentation.