Append text to Multiline column

Hi,

I have a Note field created as below in a custom Task List.

<Field Type="Note" DisplayName="User Comments" NumLines="6" RichText="TRUE" RichTextMode="FullHtml" IsolateStyles="TRUE" Sortable="FALSE" Required="FALSE" ID="{guid}" StaticName="MyUserComments" Name="MyUserComments" AppendOnly="TRUE" Group="TestGroup" />

The column is getting created. When i first add some data to it, it is getting saved.

When i reopen the column and add some more data, the newly added data is not getting saved. Only the old data is shown

I am doing this programmatically.

Below is the code I have used:

_ListItem["MyUserComments"] = txtComments.Text;

How to append the newly added text to existing text in multi-line field programmatically?

Thanks