Getting Invalid Pipline Operator in MongoDB intellishell studio 3t - mongodb

Question

I am running the following query, trying to append a new element to an existing array, but it keeps giving me invalid pipeline operator whenever i user $push or $addToSet.
What can i do to make the new field i am adding add the new element?
What am I doing wrong?
Here is my code:
db.getCollection("Branch").aggregate([{
$match: {
"company.id": "439424454545436160"
}
},
{
$lookup: {
from: "Desk",
localField: "_id",
foreignField: "branch.id",
as: "desks"
}
},
{
$unwind: "$desks"
},
{
$lookup: {
from: "Service",
localField: "_id",
foreignField: "branch.id",
as: "services"
}
},
{
$addFields: {
"desk-services": {
$map: {
input: "$services._id",
as: "sv",
in: {
"$cond": {
if: {
"$eq": ["$desks.services.id", "$$sv._id" ]
},
then: {
$addToSet: "$$sv"
},
else: {
}
}
}
}
}
}
},
{
$project: {
"_id": 1,
"name": "$desks.name",
"label": "$desks.label",
"services": "$desks.services",
"branch": "$name",
"active": 1,
}
},
{
$sort: {
"branch": 1,
"desk": 1,
}
}
])
UPDATE:
output without mapping:
Expected output:
i want the below output but with a new element that is in the service collection