How to write alpha to destination in a fixed function shader...

How do I cause a fixed function shader, to write alpha to the destination (for use in a render texture) ?
I have previously added this to standard vertex/pixel shaders…
#pragma surface surf Lambert keepalpha

but that is bad syntax for a fixed function shader…
What is the equivalent “keepalpha” command for a fixed function shader please ?
or is it not possible ?
Do I need to re-write this shader as a typical vertex/pixel shader?

Cheers

gave up. re wrote it as vertex + pix shader