I think its enough to have this in the event template:

Nginx certificate will expire in 14 days for: %<instance> on server %n

%<instance> is the certificate name in my case
and %n is the node or object name in netxms.

Thank you so much for the input though.

When developing your script, you can run it via Execute Script on the node where you have your table DCI. However, there's a difference - when script is ran automatically on threshold violation, variables $event, $dci will be set - you can use these to get parameters of event or id of the DCI. When running via Execute Script, these variables will not be set, so you need to modify your script slightly.

Here's a brief example:

//t = GetDCIValue($node, $dci.id); // for use in script called on threshold violation
t = GetDCIValue($node, 13741); // for testing - use via Execute Script on node

//row = $event.getParameter(4); // for use in script called on threshold violation
row = t.findRowIndexByInstance("/sys"); // for testing

return t.get(row, 1);

You can give some more information, what exactly cell of your table you want to extract, then I can give some more examples.