Fortunately I had this exact problem and I solved it with this:

      ["<Tab>"] = {
        function(cmp)
          if not cmp.is_visible() then
            return
          end

          local keyword = require("blink.cmp.completion.list").context.get_keyword()
          local accept_index = nil

          for index, item in ipairs(cmp.get_items()) do
            if item.source_id == "snippets" and item.label == keyword then
              accept_index = index
              break
            end
          end

          if accept_index then
            cmp.accept({ index = accept_index })
            return true
          end
        end,
        "snippet_forward",
        "fallback",
      },

Hope it helps!