How to get the Forward voltage of a tachometer?
I'm not sure what you mean by "forward voltage". That's a term usually used to describe the voltage drop over diodes.

The device you linked appears to be an all-in-one solution. You'll just need to power it using a wall wart between 8 and 15v. Anything > 300ma should be fine.

Here's a wiring diagram one of the reviewers made: