Hey there!
This is actually quite simple to do.
//register jump bindings
InputComponent->BindAction("Jump", IE_Pressed, this, &AMyCharacter::OnStartJump);
InputComponent->BindAction("Jump", IE_Repeat, this, &AMyCharacter::OnStartJump);
InputComponent->BindAction("Jump", IE_Released, this, &AMyCharacter::OnStopJump);
As you can see, on the second line you can bind the jump starting function to IE_Repeat as well. This causes it to be repeatedly fired when you hold the key down