Replace a JSON value with string of a TAIL node

Hi Community,
I´m a fresh man in NR and need your help.
I try to use a string from my TAIL node and use this string to replace a placeholder in my JSON Object which I will use as a http POST request.
I stuck with the replacing. I have no idea how to replace the JSON value ("#????") with my value from my "tail-string" (e.g. DC12)

Thx !!

[{"id":"44f39358.4f8b4c","type":"tab","label":"Flow 1","disabled":false,"info":""},{"id":"b0be09cb.e33aa8","type":"http request","z":"44f39358.4f8b4c","name":"Agilox Workflow","method":"POST","ret":"obj","paytoqs":"ignore","url":"http://141.73.250.4:8100/workflow/0","tls":"","persist":false,"proxy":"","authType":"","x":560,"y":300,"wires":[["9c9bd940.eb1fa8","59e8e1ec.845ec"]]},{"id":"9c9bd940.eb1fa8","type":"debug","z":"44f39358.4f8b4c","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":810,"y":260,"wires":[]},{"id":"59e8e1ec.845ec","type":"file","z":"44f39358.4f8b4c","name":"Agilox Order Logfile","filename":"C:\\Users\\xxxx\\Desktop\\Agilox.txt","appendNewline":true,"createDir":false,"overwriteFile":"false","encoding":"none","x":830,"y":320,"wires":[[]]},{"id":"da10ec19.d874a","type":"tail","z":"44f39358.4f8b4c","name":"agilox","filetype":"text","split":"[\\r]{0,1}\\n","filename":"C:\\\\Users\\\\xxx\\\\Desktop\\\\Node-Red Test\\\\Agilox.txt","inputs":0,"x":90,"y":400,"wires":[["648d22c8.cccd7c","cf308e72.2fb21"]]},{"id":"648d22c8.cccd7c","type":"debug","z":"44f39358.4f8b4c","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":810,"y":400,"wires":[]},{"id":"cf308e72.2fb21","type":"change","z":"44f39358.4f8b4c","name":"Destination zufügen","rules":[{"t":"set","p":"payload","pt":"msg","to":"{    \"SOURCE\": **#??????**?,    \"DESTINATION\": \"DC16\",    \"PRIORITY\": 50,    \"userdata\": {        \"host_orderid\": 3541387    }}","tot":"json"}],"action":"","property":"","from":"","to":"","reg":false,"x":340,"y":300,"wires":[["b0be09cb.e33aa8"]]}]