embed subquery result as csv inside another as json in mssql

I must provide query results in json format, one of the data strings is an array/list. I can provide the query and get a result but the system ingesting it requires it in a slightly different format, can anyone help with my query to achieve this.

We are querying a publications database, two tables, [publications] and a table linking them to user accounts [publications-users] a single result is shown below:

{
    "_publication": {
        "identifier": "a2v3h2t8e7w",
        "authors": "Smith M; Jones G; Atkinson R",
        "title": "A publication about something.",
        "researchers": [
            {
                "uid": "user658"
            },
            {
                "uid": "user663"
            }
        ]
    }
}

using code something like:

SELECT P.[id] as '_publication.identifier',
  P.[authors] as '_publication.authors',
  P.[title] as '_publication.title',
  ( SELECT PU.[userid] FROM [publications-users] PU WHERE P.[id]=PU.[pubid] FOR JSON PATH) as '_publication.researchers'
FROM [publications] P
FOR JSON PATH;

but the software receiving the json requires the researchers bit to be formatted as a CSV string:

{
    "_publication": {
        "identifier": "a2v3h2t8e7w",
        "authors": "Smith M; Jones G; Atkinson R",
        "title": "A publication about something.",
        "researchers": [
            {
                "user658","user663"
            }
        ]
    }
}

I found STRING_AGG but this not available in Azure Data Studio

'string_agg' is not a recognized built-in function name.

Suggestions for best way to achieve this output please? It's probably really simple, I just can't get it!