Hello Madhu,
Please use the below and check for Create operation
/sap/opu/odata/sap/ZXXXXXXXXX_F4M_SERVICES_SRV/FormSet
Inside slug pass your Matid and Form Tempid which are keys.
Since you have 2 Keys here. I am not sure how will you pass. May be for time being to test you can concatenate as Matid_FormTempid like 100000000000000000_100000000000000001.
Inside code split on ' _ ' and get the keys.
Later you can check and see if you can define a single Key so that you can pass it inside slug.
I assume your are already passing correct Content-Type also.
Regards,
Ashwin