ANY HELP WITH WRITING BACK TO An EXISTING SUBFORM RECORD

Tried this so many ways...

How can I write this back to this "subscriber list"-FORM with Field List "Demo" or ID "3412703000009959015" ? My Inventory List already is setup with a list of contacts and I want to write more records from another form with email fields. The code below writes to my lists as new entries.

subformCollection = Collection();

row1 = Inventory_Subscriber_list.SubForm();

row1.Account_Number=input.Account;

row1.Email=input.Email_1;

subformCollection.insert(row1);

newRecordID = insert into Inventory_Subscriber_list

[

Added_User=zoho.loginuser

Inventory_List=input.B_Stock_List

SubForm=subformCollection

];