I don ‘t see a clean way to create a scheduled task to run immediately, but you just create one in the distant future, and then send another schtask command to run it:

schtask /run /tn testtask

SCHTASKS /Create [/S system [/U username [/P password]]]
[/RU username [/RP password]] /SC schedule [/MO modifier] [/D day]
[/I idletime] /TN taskname /TR taskrun [/ST starttime] [/M months]
[/SD startdate] [/ED enddate]

Description:
Enables an administrator to create scheduled tasks on a local or
remote systems.

Parameter List:
/S system Specifies the remote system to
connect to. If omitted the system
parameter defaults to the local
system.

/U username Specifies the user context under
which the command should execute.

/P password Specifies the password for the given
user context.

/RU username Specifies the user account (user
context) under which the task runs.
For the system account, valid values
are “”, “NT AUTHORITY\SYSTEM” or
“SYSTEM”.

/RP password Specifies the password for the user.
To prompt for the password, the value
must be either “*” or none.
Password will not effect for the
system account.

/SC schedule Specifies the schedule frequency.
Valid schedule types: MINUTE, HOURLY,
DAILY, WEEKLY, MONTHLY, ONCE,
ONSTART, ONLOGON, ONIDLE.

/MO modifier Refines the schedule type to allow
finer control over schedule
recurrence. Valid values are listed
in the “Modifiers” section below.

/D days Specifies the day of the week to run
the task. Valid values: MON, TUE,
WED, THU, FRI, SAT, SUN and for
MONTHLY schedules 1 – 31 (days of the
month).

/M months Specifies month(s) of the year.
Defaults to the first day of the
month. Valid values: JAN, FEB, MAR,
APR, MAY, JUN, JUL, AUG, SEP, OCT,
NOV, DEC.

/I idletime Specifies the amount of idle time to
wait before running a scheduled
ONIDLE task.
Valid range: 1 – 999 minutes.

/TN taskname Specifies a name which uniquely
identifies this scheduled task.

/TR taskrun Specifies the path and file name of
the program to be run by this
scheduled task.
Example: C:\windows\system32\calc.exe

/ST starttime Specifies the time to run the task.
The time format is HH:MM:SS (24 hour
time) for example, 14:30:00 for
2:30 PM.

/SD startdate Specifies the first date on which the
task runs. The format is
“mm/dd/yyyy”. /ED enddate Specifies the last date when the task
should run. The format is
“mm/dd/yyyy”. /? Displays this help/usage.

Modifiers: Valid values for the /MO switch per schedule type:
MINUTE: 1 – 1439 minutes.
HOURLY: 1 – 23 hours.
DAILY: 1 – 365 days.
WEEKLY: weeks 1 – 52.
ONCE: No modifiers.
ONSTART: No modifiers.
ONLOGON: No modifiers.
ONIDLE: No modifiers.
MONTHLY: 1 – 12, or
FIRST, SECOND, THIRD, FOURTH, LAST, LASTDAY.

Examples:
SCHTASKS /Create /S system /U user /P password /RU runasuser
/RP runaspassword /SC HOURLY /TN rtest1 /TR notepad
SCHTASKS /Create /S system /U domain\user /P password /SC MINUTE
/MO 5 /TN rtest2 /TR calc.exe /ST 12:00:00
/SD 10/20/2001 /ED 10/20/2001 /RU runasuser /RP SCHTASKS /Create /SC MONTHLY /MO first /D SUN /TN game
/TR c:\windows\system32\freecell
SCHTASKS /Create /S system /U user /P password /RU runasuser
/RP runaspassword /SC WEEKLY /TN test1 /TR notepad.exe
SCHTASKS /Create /S system /U domain\user /P password /SC MINUTE
/MO 5 /TN test2 /TR c:\windows\system32\notepad.exe
/ST 18:30:00 /RU runasuser /RP *
SCHTASKS /Create /SC MONTHLY /MO first /D SUN /TN cell
/TR c:\windows\system32\freecell /RU runasuser