I will define some group-like element acting on the free-fermionic space in terms of 3-point Riemann-Hilbert problem. Then I will show that this operator is actually the $W_n$ primary field (in some cases it is the only definition of the primary field). Also I will show that the vacuum expectation value of n such fields is the n-point isomonodromic tau-function. This gives the proof of the isomonodromy-CFT correspondence in the general situation.