A potentially dangerous Request.Form value was detected from the client - ASP.NET MVC

In MVC you would use the ValidateInput(false) attribute.

You then need to sanitize your inputs, e.g. with something like this (built in to ASP.NET 4.5+; use NuGet package for earlier).

In MVC 3 and later, you can also use the [AllowHtml] attribute. This attribute allows you to be more granular by skipping validation for only one property on your model.

https://docs.microsoft.com/en-us/dotnet/api/system.web.mvc.allowhtmlattribute?view=aspnet-mvc-5.2

Just place this attribute: [ValidateInput(false)] on the action method on the controller that handles the form post.