What's the code for "print string" node in bp?

For the print string node you would most likely want to use the AddOnScreenDebugMessage function.

Docs: UEngine::AddOnScreenDebugMessage | Unreal Engine Documentation

Example:
void AddOnScreenDebugMessage
(
    -1, (always on top)
    2.f, (Display for two seconds)
    FColor::Green,
    TEXT("This is an example message!")
)