LSP, as a general framework, doesn’t do any exact job, it just obeys server responses. You would have to check the document of your used language server (e.g., maybe clangd, which actually uses clang-format for formatting).
After googling clangd Allman, you can quickly find this https://gist.github.com/maddouri/f9239a941ea753bafeb658e330ae92ab#file-clang-format-L20. So it’s likely can be done by creating a .clang-format file with that line included in your project root. Or maybe you want to use the whole file with some minor modifications since it claims it’s for Allman-style formatting. .clang-format generator: https://zed0.co.uk/clang-format-configurator/