Good, in the Index file I have an Input in which I want to enter a text string and pass it to an ActionResult of a controller, and from this I call a method of another class, but I do not know how to do it.
In MVC there is what is called Model Binding, among other things, what it does is that for each element of a form, these are sent to the action method (Action Method). There are different ways to receive the elements of the form in an Action Method, being in this case, if you have an input with a specific name, you can catch this value if you expect a parameter with the same name. So in your case, what you need to do is wait for a field with the name of textCliente
[HttpPost]
public ActionResult registrarClient(string textCliente)
{
...