How to Get Request Post Params from Rest Webapi Magento 2?

You can Retrieve the Rest Webapi Request Parameters while you working with the REST API in Magento.

Magento has the OOTB Webapi module that is dealing with Rest API functionality.

Useful Class, Magento\Framework\Webapi\Rest\Request

<?php
namespace Jesadiya\RestParams\Model;

use Magento\Framework\Webapi\Rest\Request;

class GetRestParam
{
    /**
     * @var Request
     */
    protected $request;

    public function __construct(
        Request $request
    ) {
        $this->request = $request;
    }

    public function getParamValue()
    {
        $filter = $this->request->getParam('paramerer_name');
    }
}

If you are working for the REST API, Passing a parameter that needs to expose using Magento code, try with the given code to get the value of each param in REST API,

in the function, ‘paramerer_name’ is your actual parameter name to fetch the value from the request.

If you want to check all the parameters for the Rest API,
$this->request->getParams(); and print the list of key-value for the request parameter.

Fetch data from HTTP Request body using,

$this->request->getBodyParams();

This is the simple way to get the request parameter value in Magento with REST API.