Use multiple statements in if condition SQL Server

Hi , i want to insert value into table in stored procedure and return true or false based n condition

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE spx_CheckUserAvailability
      @UserName VARCHAR(50)
AS
BEGIN
      SET NOCOUNT ON;
      IF NOT EXISTS
            (SELECT * FROM Users
             WHERE UserName = @UserName
            )
            SELECT 'true'
      ELSE
            SELECT 'false'
END
GO

If output is true then it insert into user table as username into users table and then return true else false