This piece of code will let you create SharePoint  2010 web application programmatically enabling FBA.

SPWebApplicationBuilder webAppBuilder = new SPWebApplicationBuilder(SPFarm.Local);

                   webAppBuilder.Port = portNo;
                   webAppBuilder.ApplicationPoolId = "IIS application ID";
                   webAppBuilder.IdentityType = IdentityType.SpecificUser;
                   webAppBuilder.ManagedAccount = mngAcc;
                   webAppBuilder.RootDirectory = new DirectoryInfo("physical path of web application"));
                   webAppBuilder.ServerComment = "Server comment";
                   webAppBuilder.UseNTLMExclusively = true;
                   webAppBuilder.AllowAnonymousAccess = false;
                   webAppBuilder.CreateNewDatabase = true;
                   webAppBuilder.DatabaseServer = "SQL Server name";
                   webAppBuilder.DatabaseName = "Database name";

                   if ("SQL Authentication is selected")
                   {
                       webAppBuilder.DatabaseUsername = "username";
                       webAppBuilder.DatabasePassword = "password";
                   }

                   newApplication = webAppBuilder.Create();

                   SPFormsAuthenticationProvider fbaAuthProvider = new SPFormsAuthenticationProvider(membershipProvider, roleManager);

                   newApplication.UseClaimsAuthentication = true;
                   newApplication.IisSettings[SPUrlZone.Default].AddClaimsAuthenticationProvider(fbaAuthProvider);
                   newApplication.Update();
                   newApplication.Provision();