If I ordered Final Cut Pro with MacBook Pro, do I have to purchase it again for my iMac?

You can install FCP on as many Macs as you control. Login to the App Store with the Apple ID you used to purchase FCP and click on your Apple ID/name at the bottom left of the window. From here you can download FCP.

I haven't purchased bundled software with a Mac, so it's possible there's an Accept step with your new MacBook Pro you need to do first.

Accepting preinstalled applications in the App Store

Accept preinstalled Pro Apps on your new Mac - Apple Support

Do yourself a favor and buy it separately.

This way the purchase is forever tied to your Apple ID.

If you buy it with a mac, you should still be able to use it on any macs that you own and control, but you first have to "accept" this purchase into your Apple ID.

This sometimes works just fine, but often people aren't aware of it and later get into trouble when they want to update the application or use it in another mac.

I understand that sometimes one has to do it this way. In that case, "accept" the purchase as soon as you get it.