http://localhost:9002/api/graphiql

mutation createJoin($input: JoinUpdateInput!) {
  createJoin(input: $input) {
    urn
    joinId
  }
}


Variables:

{
  "input": {
    "properties": {
      "name": "testName 2156",
      "dataSetA": "urn:li:dataset:(urn:li:dataPlatform:snowflake,DQM_DEV_NXGEXTDEV_DB.QTNGDEV.LINKER_FIELD_DEFS,DEV)",
      "datasetB": "urn:li:dataset:(urn:li:dataPlatform:snowflake,DQM_DEV_NXGEXTDEV_DB.QTNGDEV.LINKER_FIELD_DEFS_NEW_NEXT,DEV)",
      "joinFieldmappings": 
        {
          "details": "Details of join 2156",
          "fieldMapping": [
            {
              "afield": "FIELD_NAME",
              "bfield": "PRDT_TYP"
            },
            {
              "afield": "FIELD_ORDER",
              "bfield": "FIELD_LENGTH"
            }
          ]
        }
      
    }
  }
}

Output:
{
  "data": {
    "createJoin": {
      "urn": "urn:li:join:d8bcde23-2043-4cb4-a1e0-ad3b580cccf6",
      "joinId": "d8bcde23-2043-4cb4-a1e0-ad3b580cccf6"
    }
  },
  "extensions": {}
}