How can I use a service outside of Startup.cs in my ASP.NET Core Application?

I has an ASP.NET Core application that serves as a sort of "job scheduler", thus I need to have the application run a cron job for configured jobs. I figured out a method to do this, but unfortunately, it only works in the Startup.cs file using the IServiceCollection abstract class. Due to the nature of this application, I need to be able to add a cron job in my controller when a post request is made.

Here is how I am adding a cron job inside Startup.cs:

public void ConfigureServices(IServiceCollection services)
        {
                // Basic config code above here

                services.AddCronJob<CronJob>(c =>
                {
                    c.TimeZoneInfo = TimeZoneInfo.Local;
                    c.CronExpression = @"" + cronEx;
                    Console.WriteLine(c.CronExpression);
                });
            }
        }

Here is my API controller and what I am trying to do:

// POST: api/Jobs
// To protect from overposting attacks, please enable the specific properties you want to bind to, for
// more details see https://aka.ms/RazorPagesCRUD.
[HttpPost]
public async Task<ActionResult<Job>> PostJob(Job job)
{
    _context.Job.Add(job);
    await _context.SaveChangesAsync();

    services.AddCronJob<CronJob>(c =>
    {
          c.TimeZoneInfo = TimeZoneInfo.Local;
          c.CronExpression = @"" + job.CronExpression;
    });

    return CreatedAtAction("GetJob", new { id = job.Id }, job);
}

I'm not totally sure if this is possible, but any insight would be helpful.