When using SQL Passthrough and executing a SP with multiple parameters including an OUTPUT parameter, is it possible to pass the parameters by reference so that that some of the parameters can be omitted and left to their default values while still getting the OUTPUT parameter?

E.g. if a SP is defined as:

CREATE PROCEDURE MyProc @P1 varchar(6), @P2 int, @P3 int OUTPUT
AS ...

A typical passthrough execution setup and string might be:

PRIVATE lnP3
lnP3 = 0
lcSQL = [EXEC MyProc 'ABCD', 1, ?@lnP3]

Now suppose I want to pass by reference omitting @P2 and allowing it to default to its default value in the SP but still get the output parameter back. Is this possible and if so, what would the syntax be? I have tried things like:

PRIVATE lnP3
lnP3 = 0
lcSQL = [EXEC MyProc @P1 = 'ABCD', @P3 = ?@lnP3]

but this does not seem to work. If it is possible I'm missing something in the syntax.

Thanks,
Bill