Go to the Tests section in Polygon and add a test case where the expected output is what participants need to produce to receive an "Accepted" verdict.

If you want multiple specific outputs to be accepted, for example both agent_p and helloworld123, you will need to create a custom checker in the Checker section of Polygon.

Example code below:

#include "testlib.h"

int main(int argc, char* argv[]) {
    registerTestlibCmd(argc, argv);
    std::string participantOutput = ouf.readToken();
    ouf.readEof();
    if (participantOutput == "agent_p" || participantOutput == "helloworld123") {
        quitf(_ok, "Output is correct: %s", participantOutput.c_str());
    } else {
        quitf(_wa, "Output is incorrect: %s", participantOutput.c_str());
    }
}

I've prepared output-only problems multiple times.

In the input section I usually write single sentence "No input" or "Reading data is not required" and then set up tests like that

Multiple tests in case a solution is not deterministic.

Putting "NO INPUT" and disincentivising participants from reading it might be even better than a blank input. If you really want to make a test with empty input you can uncheck "Tests well-formed" and then "Add Test" > "from the files" and upload empty file.

If you want to include a sample into problem statement to show output format, but at the same time you want to hide the real answer to the problem you can

1. "Edit" a test

2. check "Use in statements"

3. "If you want to specify custom content of input or output data for statements click here"

4. provide statement-only output

5. uncheck "Verify output for statements"

Also you can implement custom checker that will directly compare solution output to the right hardcoded answer.