spindle with mesa 7i76e and LinuxCNC

What do you mean by 10v power supply ?
If you are referring to the analog control input on the VFD, then almost all VFD will have a 10V output, GND, and Vref outputs/inputs on the same terminal block, and those should be wired to the Mesa spindle termial.
You can use it without encoder,
You will have to set the VFD to be controlled externally, so check the user manual.
Some VFD will also need another pin to enable the drive in CV and another for CCV if you need to reverse it. Those are usually 24V tolerant inputs or require to be brought to GND, again check manual.

The 7i76 provides a digital replacement of a potentiometer and two switches.

So, you can wire the VFD to the 7i76 exactly as the VFD manual shows wiring for a potentiometer and switches but with one very important caveat:
The switches and the potentiometer on the 7i76 are polarity sensitive and must be connected with the correct polarity.

The switches are also configured for "on" and "dir" rather than "forward" and "reverse" (ie forward = 10 and reverse = 11 rather than fwd = 10 and rev = 01)
If you can't configure the VFD to work this way then a bit of cunning may be needed. (But note that many machines have absolutely no need to reverse the spindle. Most routers, for example)

Arro wrote:

I tried a quick test measuring pin 2 on TB4 (spindle output), but I have no signal (0V)
spindle-.output signal on HAL is increasing with LinuxCNC signal, 101..102..103

It is an (isolated) digital potentiometer, it needs to be connected to the +10V and 0V of the VFD or there will be no voltage on any of the pins.

When I said "connect it like a potentiometer" I meant "connect it like the VFD manual shows a potentiometer to be connected"