I was able to find a class which is responsible for a such refactoring
/netbeans/java/java.hints/src/org/netbeans/modules/java/hints/introduce/IntroduceExpressionBasedMethodFix.java

That code is quite hard to understand. So far, I am bit lost. What I discovered is, that no input params is found. I suspect something is nok in ScanStatement, but not sure what.

So a bit of progress. It seems, all what is needed is to put references to variables into "usedLocalVariable". I made a simple POC, I added the "tempForceUsedLocalVariable" to be able to change the behavior while debugging netbeans:

ScanStatement.java

    @Override
    public Void visitIdentifier(IdentifierTree node, Void p) {
        Element e = info.getTrees().getElement(getCurrentPath());
        if (e != null) {
            boolean tempForceUsedLocalVariable = false;
            if (tempForceUsedLocalVariable){
                usedLocalVariables.put((VariableElement) e, true);
            }

After this modification, input parameters appeared in newly created method. So it seems it points to right direction:

    private static Supplier<String> concatSup(String first, String second) {
        return ()->first+second;
    }

Anyway, I am still a bit confused by the code itself.

I fought with code base to be able to extract lambda expression with {} but I failed.

If I extract return t + text + m1;

from

private int dummy(final String text) {
    Function<String, String> test2 = t -> {
        return t + text + m1;
    };

    return test2.apply(text).length();
}

Netbeans wrongly deduct the return type from method, whic int:

private int dummy(final String text) {
    Function<String, String> test2 = t -> {
        return inner(t, text);
    };

    return test2.apply(text).length();
}

private int inner(String t, final String text) {
    return t + text + m1;
}

I haven't found a way how to make it working so far :(

But at least it works for lambda without {}