How to calculate using mesh method when there's a current source and a resistor between two meshes
You should treat the circuit as if the current source and the components in series are not there in the first steps.
Then, relate the meshes with the current source like you use to do when the is just a current source without components in series with it.

Note that a current source can have any voltage across is, so the voltage drop across components in series with the current source is not relevant as it will not affect the circuit.
(And the current is of course dictated by the current source.)
That explains why you can leave those series components out together with the current source.