Don’t use FromBody. You’re submitting as x-www-form-urlencoded (i.e. standard HTML form post). The FromBody attribute is for JSON/XML.
You cannot handle both standard form submits and JSON/XML request bodies from the same action. If you need to request the action both ways, you’ll need two separate endpoints, one with the param decorated with FromBody and one without. There is no other way. The actual functionality of your action can be factored out into a private method that both actions can utilize, to reduce code duplication.