In order to take a challenge for a specific company, you need to contact the company directly. You cannot access these challenges by signing up for HackerRank For Work. We host the challenges, however, the only way to access them as a Candidate is to be invited by the hiring company.
For coding practice and events, please visit the HackerRank Community site where we host challenges, coding contests, recruiting events, tutorials, jobs, and practice tests.
Refer to the Introduction to HackerRank's coding community topic for detailed information.