IMO it doesn't take long before it becomes easier to insert these fields
directly, e.g.
a. put the insertion point where you want the field
b. press ctrl-F9 to insert a pair of "special field braces" to give you

{ }

c. type IF "" = "" "the text you want" "" in between to give you

{ IF "" = "" "the text you want" "" }

d. put the insertion point between the first "" and press F9 again

{ IF "{ }" = "" "the text you want" "" }

e. type MERGEFIELD "Mail_ID"

{ IF "{ MERGEFIELD "Mail_ID" }" = "" "the text you want" "" }

 f. either copy/paste the { MERGEFIELD "Mail_ID" } field between the final
"" or repeat (d) and (e) but putting the insertion point between the final
"" to give you

{ IF "{ MERGEFIELD "Mail_ID" }" = "" "the text you want" "{ MERGEFIELD
"Mail_ID" }" }

You can drop the quotes around Mail_ID if it doesn't have spaces and so on.
You can usually drop the quotes around the { MERGEFIELD Mail_ID } fields as
well, but I prefer to keep them.

Use Alt-F9 to toggle between field codes and results/preview, and sleect and
press F9 to update a particular field's result as necessary.

Peter Jamieson