When saving to an organization, selecting at least one collection is required. If there’s only one collection available, it would be convenient if the checkbox came already selected.
I do agree this would be an excellent feature, possibly even being able to have default folder/collection for each client similar to how vault timeout options are client independent.
This would be handled on the client side though, as Vaultwarden only builds out a compatible backend server.
There appears to be a few feature requests for this on the official bitwarden forums, may go upvote there and add your ideas to help get it in front of the developers.
While we could check if there is only one collection and auto select it, I’m not fond of defeating to much from upstream on these items. Since they could cause strange issue in the future.
Since there already is a feature request upstream, i would go for that.