Web Api HTTPPost not accepting int

1.Your [HttpPost] expects a int but from body you are passing a json object. you should pass json string like below. No need to mention parameter name

2.you should use [FromBody] as below

[HttpPost]
    public void UpdateMainVerified([FromBody] int mainid)
    {

    }

this link explains it well

https://docs.microsoft.com/en-us/aspnet/web-api/overview/formats-and-model-binding/parameter-binding-in-aspnet-web-api

Set FromBody Attribute.

To force Web API to read a simple type from the request body, add the [FromBody] attribute to the parameter.

For details Link

[HttpPost]
[Route("api/UpdateMainReversed")]
public IHttpActionResult UpdateMainVerified([FromBody] int mainId)
    {  ....}