To add a Video Link field to a form, first you must ensure you have created the Video Link property in the object that you are using. For this you also need the Video Link data type which you have created. Then, in build studio for a page, you must use that object as a base for a form widget. In the ‘Fields’ tab of the form widget, you will find your Video Link property which you can double click on to add it to your form. Please add a submit button to the form at this point. I would also suggest adding an information widget based on the same object just below your form widget, that contains your video link field, to check if the link saves and displays the video player correctly. Publishing and accessing this page normally should present you with a field that will accept your video link and upon saving, it will show the embedded video in the information widget.
Please ensure, when you are adding the video link, that you do not have any spaces before and after the link as this can sometimes behave funny. The link also needs to be start with “https://”. Let me know how you get on.