Creating a benchmark assessment is a district-only feature.
Click on Admin, then hover over Assessments and select Benchmarks.
Click on Add Benchmark.
Select District Benchmark as the Assessment Type. When you select District Benchmark, the Standard will be Multi-Standard by default.
If you want teachers to be able to remove the benchmark from their trackers, select the Allow Teachers to delete From Trackers check box. If you do not check this box, teachers won't be able to remove the assessment from trackers where you've made it available.