Can you post a code snippet so I have an idea what are you trying to do?
If I understand correctly, you want to display "Guild Trader" and then later update it to "Guild Trader (Guild Name)".
So you can either make two different strings:
Lua Code:
ZO_CreateStringId("PIN_GUILDTRADER_NONAME", "Guild Trader")
ZO_CreateStringId("PIN_GUILDTRADER_GUILDNAME", "Guild Trader (<<1>>)")
local text = guildname ~= nil and zo_strformat(PIN_GUILDTRADER_GUILDNAME, guildName) or GetString(PIN_GUILDTRADER_NONAME)
Or just concatenate strings:
Lua Code:
ZO_CreateStringId("PIN_GUILDTRADER", "Guild Trader")
local text = GetString(PIN_GUILDTRADER) .. (guildName ~= nil and guildname or "")