How can I simulate FR4 microstrip substrate ?

You just enter its properties in the Basic Parameters dialog.

Tell us which software do you use to simulate.

i use from ie3d software and i say method create substrate in this software.

@sahar20:
To assign material for 3D objects in HFSS (I guess you're using).
Chose your desired object, right click and select 'Assign material'
Next steps are easy.
nice weekend!