How do I append data instead of replacing it?

I understand how to fetch and change records from a stateless form now, but

how can you "append" to a text field from a stateless form? Right now this works to replace:

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if (input.Service_Date != null)
{
temp = Tracker [CLIENTID == input.CLIENTIDLOOKUP];
temp.Code_10_Record = input.Service_Date.toString();
}

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but I want something like this, which will not work:

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if (input.Service_Date != null)
{
temp = Tracker [CLIENTID == input.CLIENTIDLOOKUP];
insert into Tracker
[
Code_10_Record = "\n" + input.Service_Date.toString();
]
}