

Are you trying to filter known communities by their instance? That is not supported by the Lemmy API, so a client can’t do that without depending on some other third party API.
~~https://join-lemmy.org/api/main#tag/Miscellaneous/operation/Search~~
edit: A client can do it, but I don’t think any client has that feature.
See my edit. It is possible, but the client needs to implement it.
If you think it would be useful, open a feature request in your client’s repository. It can be done by getting the list of all the communities in your instance and searching by instance domain.