Microsoft planned to release the first beta test for Halo: The Master Chief Collection on PC in April but that is no longer happening. Developer 343 Industries released a post on why the delay is necessary. 343 is currently finalizing the process of the technical ability for Steam users to acquire the flight making sure they are ready for the first public test.
Halo: The Master Chief Collection Beta Delayed - GS News Update
Unfortunately, the beta for Halo: The Master Chief Collection is being delayed