Trying to set AssignedTo to in a Work item - azure-devops-rest-api

Question

I am trying to set the AssignedTo in a PowerShell script, using the REST API.
The user already has a bunch of work items assigned, and has been in the system for several month.
I have tried using the Displayname, email (same as upn), and the link to the identity. All gives the same result:
Invoke-WebRequest : {"$id":"1","customProperties":{"ReferenceName":"System.AssignedTo"},"innerException":null,"message":"The identity value 'xxx#xxx.com' for field 'Assigned To' is an unknow
n identity.","typeName":"Microsoft.TeamFoundation.WorkItemTracking.Server.WorkItemFieldInvalidException, Microsoft.TeamFoundation.WorkItemTracking.Server","typeKey":"WorkItemFieldInvalidExce
I use the following script:
$tfsToken = "MYPAT"
$token = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes(":$($tfsToken)"))
$defaultColletion = "default_Collection"
$project = "Agile"
$childID = "63573"
$identityUpn = "xxx#xxx.com"
$childuri = ("http://tfs:8080/tfs/" + $defaultColletion + "/" + $project + "/_apis/wit/workitems?ids=" + $childID + "&expand=relations&api-version=5.0")
$header = #{authorization = "Basic $token"}
$childresult = Invoke-WebRequest -Uri $childuri -Method Get -Headers $header -ContentType "application/json-patch+json"
$childObj = $childresult.Content | ConvertFrom-Json
$updUrl = ($childObj.value.url + "?api-version=5.0")
$ownerUpd = #{}
$ownerUpd.Add("path","/fields/System.AssignedTo")
$ownerUpd.Add("op","add")
$ownerUpd.Add("value",$identityUpn)
$json = ConvertTo-Json #($ownerUpd) -Depth 5
$result = Invoke-WebRequest -Uri $updUrl -Headers $header -Method Patch -Body $json -ContentType "application/json-patch+json"

Answer 1

You can set into System.AssignedTo exactly what you see in Assigned To field in work item: Display Name of a user. You can find example here Update an identity field by display name:
[
{
"op": "add",
"path": "/fields/System.AssignedTo",
"value": "Jamal Hartnett"
}
]
Additional examples:
Update an identity field by distinct display name
Update an identity field by IdentityRef

Answer 2

That user already has work item assigned to him right?
So the first step in your case is to find what his unique name is under the AssignedTo field by using the below "Get Work Item Details API":
https://dev.azure.com/{organization}/{project}/_apis/wit/workitems/{id}?api-version=5.1
Then, use the same API you've used before but this time, provide the unique name value that you got from the previous API response.
[
{
"op": "add",
"path": "/fields/System.AssignedTo",
"value": "" # His unique name goes here
}
]
I would've commented this asking if you've tried using his unique name but I still dont have the sufficient points to post the comment.

Answer 3

If you have the Name and E-mail of the user you can use both in the format like when you send an e-mail. Example:
[
{
"op": "add",
"path": "/fields/System.AssignedTo",
"value": "user name <xxx#xxx.com>"
}
]