Welcome to the community!
So, in order to perform this, you will need to make your request a Postman variable, whether it be global, environmental, or a collection variable.
Then when you’ve done that, you will have access to that variable in your pre-request section. There you can calculate your signature, then modify the variable to include the signature, or include the signature as a new variable in your request body.
So, what you would put in your request body section of the Postman Request is as follows:
This of course depends on which method you do above, but when the request is sent, the variables will be replaced with those actual value(s), and you should get your desired behavior. If you know how you’re calculating your signature, I can show you an example script of how you would achieve this.