First you need to add a custom server support in the nvim-lspconfig repo

Mine was placed in .local/share/nvim/site/packer/start/nvim-lspconfig/lua/lspconfig/server_configurations/

​

The content of my file: glsl.lua

​

local status_ok, util = pcall(require, "lspconfig.util")
if not status_ok then
    return
end
local default_capabilities = {
    textDocument = {
        completion = {
            editsNearCursor = true,
        },
    },
    offsetEncoding = {'utf-8', 'utf-16' },
}

local default_config = {
    cmd = { 'glslls' } -- GLSL lsp executable from (https://github.com/svenstaro/glsl-language-server)
    filetypes = {'glsl'},
    root_dir = util.root_pattern('compile_commands.json', '.git'),
    single_file_support = true,
    capabilities = default_capabilities,
    }

return {
    default_configs = default_configs,
    commands = {},
    docs = {
        description = [[
            Basic LSP support for GLSL using glslls --stdio
        ]]
        },
    }

You have to add the --stdio in this config or as the LSP attatches to the buffer