golang，如何执行需要用户输入的命令

I want to execute perforce command line "p4" from Go to do the login job. "p4 login" require user to input password.

How can I run a program that requires user's input in Go?

The following code doesn't work.

err  = exec.Command(p4cmd, "login").Run()
if err != nil {
    log.Fatal(err)
}

From the os/exec.Command docs:

// Stdin specifies the process's standard input. If Stdin is
// nil, the process reads from the null device (os.DevNull).
Stdin io.Reader

Set the command's Stdin field before executing it.