Just call the Timer_Tick method yourself.
If you don’t want to deal with the Tick callback method’s parameters, then just put the code that was in your Timer_Tick into another method, and call that from the Timer_Tick and from just after the Timer.Start() call
As pointed out by @Yahia, you could also use the System.Threading.Timer timer, which you can set to have an initial delay to 0. Be aware though, that the callback will run on a different thread, as opposed to the callback on the Windows.Forms.Timer which runs on the UI thread. So if you update any UI controls using the System.Threading.Timer (without invoking correctly) it’ll crash.