How do I make a dash that moves in the direction of the player's movement

Link is broken, but here's the basic idea for a horizontal dash:

The first option does what you suggested- it multiplies the x and z velocities of the player, launching them based on the direction they're currently moving. (It also overrides their y velocity to be a small upwards boost- since friction with the ground sort of interrupts the dash.)

The second (arguably more helpful, but just my opinion), adds a multiplied look-angle vector to the player's current movement, counteracting or adding to their current momentum based on the direction they're looking. 

With both of these, changing the number you multiply increases the power of the dash. You can also include a y component, but only if you want your players to be able to use this as a jump boost. Also might want to add an upper limit, (using a 'max' function), just because this is going to increase in power exponentially the faster the player is going when they dash.