Well , You haven't specified the Required parameters so I am considering first two parameters as mandatory parameters
So you SP will be as below
ALTER PROCEDURE [dbo].[proc_GetUserRequests]
(
@EmpUserId INT,
@RequestTypeId INT,
@RequestStatusId INT=NULL,
@StartDate datetime=NULL,
@EndDate datetime=NULL
)
AS
BEGIN
IF @RequestStatusId = 0 SET @RequestStatusId=NULL
SELECT @StartDate = @StartDate + '00:00:00'
SELECT @EndDate=@EndDate + '23:59:59'
SELECT Id, Username, SenderAddress, ReceiverAddress, AddedOn, ModifiedOn
FROM Requests
WHERE
EmpUserId = @EmpUserId
AND RequestTypeId=@RequestTypeId
AND RequestStatusId=COALESCE(@RequestStatusId,RequestStatusId)
AND AddedOn BETWEEN COALESCE(@StartDate,AddedOn) AND COALESCE(@EndDate,AddedOn)
END