I think what your after is the ?Source=https://......
reference at the end of you list url.
Basically when you click to add a new list item the above reference is added to the end of a url so the list knows where to postback to after the user has clicked on the submit/ok button. If there is no ?source reference then the default behaviour is to return to the list. A really simple no code solution would be to create a Thank you page in a pages or site pages library to redirect to.
What I would do is create a link to the new item page but add the ?Source=your thanks you page. Normally I would do this with the Promoted links app to get the tile view, url you be:
https://yoursite/yourlist/newitem.aspx?Source=https://yoursite/pages(or Site Pages)/thankyou.aspx
HTH Stephen