How to pass a UserManager object to the Initialize class

I have a class in asp.net core 2.0 called Initialize that has a method

public async static Task SeedData(IServiceProvider serviceProvider, UserManager<ApplicationUser> userManager)
{

}

I want to call it from my Startup class as follows:

await Initializate.SeedData(app.ApplicationServices, ?);

my question is in the second parameter, how can I do it?

Thank you.