The documented use of the substring notation is as follows:

If I command

%a=John Smith;
type %[%a,"it"];

I get

John Sm

as "John Sm" is the part of the string %a that comes before the first occurrence of "it" substring.

But if I command

%a=John Smith;
%m="it";
type %[%a,%m];

I get nothing! This would be interesting if I want, for instance, to ask the user for the substring %m to look for, and in many other cases. But it seems impossible to me to get the desired result! Any idea?

%A=This is test text.;
%M=tes;
for(ii=1;ii<=%[%A]-%[%M]-1;ii++) {
	if("%[%A,ii:$(ii+%[%M]-1)]"=="%M") {
		type -a Found \x22%M\x22 in \x22%A\x22 at position $(ii);
	}
};