how do you use benchmark::RegisterBenchmark but with a fixtured benchmark that was previously defined using BENCHMARK_DEFINE_F(FixtureClass, MethodName)[BUG]

**Describe the bug**
how do you use benchmark::RegisterBenchmark but with a fixtured benchmark that was previously defined using BENCHMARK_DEFINE_F(FixtureClass, MethodName)
whenever I use the MethodName in benchmark::RegisterBenchmark I get a compilation error
**System**
Which OS, compiler, and compiler version are you using:
- OS: linux
- Compiler and version: gcc 9.4

**To reproduce**
Steps to reproduce the behavior:
1. define fixture benchmark using : BENCHMARK_DEFINE_F(FixtureClass, MethodName)
2. use the MethodName in benchmark::RegisterBenchmark

**Expected behavior**
No compilation error
it gives me : undefined reference error to MethodName