This question is regarding SharePoint Online 2013.
I have a team site setup with a custom list. I want to have a "thank you" or some sort of a confirmation page show up after the user has submitted a list item. Instead of taking the user to the list screen (from which they came), when adding a new item, I would like them to get a thank you page instead. I am familiar with using SharePoint Designer and I am sure that this will require its use.
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
You need to add the Source parameter to the url that calls the new item form. For example, the "Add new item" link at the bottom of a list uses javascript to generate a url like this:
http://yourSite/_layouts/listform.aspx?PageType=8&ListId={121A99B9-04F2-4DE9-B942-65CDD3624629}&RootFolder=
Replace that with a url that adds a source parameter with a URL to the custom page. Make sure to use your list's GUID, not the one in this sample.
http://yourSite/_layouts/listform.aspx?PageType=8&ListId={121A99B9-04F2-4DE9-B942-65CDD3624629}&Source=http://yourSite/yourLibrary/yourPage.aspx