I have a CPT called "happenings" (events) which is related to a CPT called "locations". This is for an events listing capability.
Users can enter the details of a happening through Toolset Forms and relate it to a venue ("location"). But if the location doesn't exist yet, there is no way to allow them to add a new location and relate it to the happening. Ideally, they'd enter a search in the relationship dropdown and, if no location matched, be offered an option to create a new one.
Note that this will never be possible for not-yet-published posts, as those posts cannot be related yet.
This is the same in the backend, first, you will need to save the current post, so to connect it.
That request I shared requests basically to have the same in the front end as in the backend, by behaviour.
- search
- not found returns a "create new {location}" button
- user directed to create form
- submits location
- user directed back to happening form with new location prefilled
In Many To Many Relationships, you will need Relationship Forms, those do not yet apply an API, Notifications or Expirations. Redirects can be used only to keep displaying the same page/form, redirect to a specific page, or back to where the user came from.
Since these forms do not yet apply the API you cannot have "more" on top of that, unfortunately
However, in a Many to Many relations, the process is different than in a One To Many, since you will not connect the post while creating or manipulating it, but you will actually create/manipulate the relationship when connecting posts.
You would profit a lot from the feature proposed above, I think, which would just implement the backend experience to the front end.
Conditionals to hide/show those elements should then be applied with Forms Conditions, but for that, the feature first needs to be implemented.
If you would like assistance with current possibilities outlined above, please open a new ticket so we could take a look about what would be possible (It will require custom JS to hide/show the redirect to a create Location form, likely).
With custom JS we will not be able to help but can give ideas and tips about where to look for solutions and examples.