Add a timeline, you can add a float track in the timeline and have that go from 0 to your desired speed over a specific amount of time.
Say your desired speed is 200. your timeline would look a little like this:
Remember to hit these buttons so that the timeline fits on the screen:
Here is a great video on how to do this: unreal engine 5 beginner timeline in blueprint with float and event - YouTube
Depending on the version of the engine you are using, the ‘float’ button he clicks can be found in the ‘+ Track’ button.