Open created related item form

Hello, I would like that after a specific user action on an item's form (select a certain value in a DDL or click on a button), the form to create a related item opens just as if the user used the 'new relationship' button with 'created related' selected. Is there a function in JavaScript that could do that? Especially i would like to be able to open the form to create a related item of a related item of the item (knowing the related item id) Second question is it possible to change the pre selected value Created related / pick related in the DDL or remove the 'No related' option ? Thank you. Léo