Hello @faisalfahmad, Welcome to community 
Could you please share what you have written as part of the pre-req script section?
Because based on that the expected value can be assigned to the variable of your scope and that can be used inside the body.
For example:
If I am generating the signature using random variables then I need to paste the below snippet under pre-request section,
pm.environment.set("signature", pm.variables.replaceIn('{{$randomFirstName}}'));
in you case if you are already generating the value for signature field, assign that value to the variable signature.
And then in your body you need to access the variable using {{}}
x_signature={{signature}}
Please share us more details/screenshots if you still have issue 