Form control in web part

It is the same principle, register custom tag and use it.

what will be the name space then, after creating a custom code for the form control i added it in the kentico form control and try to use that in the web part, but where the solution you have provided will not work that way. it is to add through the code file.

Should be somehting like this:

      <%@ Control Language="C#" AutoEventWireup="true" Inherits="CMSWebParts_custom_custom"  Codebehind="~/CMSWebParts/custom/custom.ascx.cs" %>
<%@ Register Src="~/CMSFormControls/Inputs/customFormoControl.ascx" TagPrefix="custom" TagName="MyCustomControl" %>

I am developing my form control in this way https://docs.kentico.com/k11/custom-development/developing-form-controls/example-developing-custom-form-controls and it should be displayed in web part in live site, is there any reference for this, the above reference is only for the form control in code type but here in the documentation it is first written in code and later added in form control to the admin panel and the form control should be used in the web part.

I'm assuming that you are asking how to use your new form control as part of a web part configuration? In the "Form controls" admin panel, make sure that you have "Controls and web parts" checked in the "Control scope" > "Show control in" section. Then, in the "Web parts" admin panel, when you are configuring the Properties of your web part, you should see your form control in the "Form control" drop down. If you still don't see it, make sure that the "Data type" of the web part field is one of the allowed data types ("Control scope" > "Use control for") in the "From control" admin.