DOELovEAB, what letter should I add to set the micro signal?

You don't need to add it, just set it directly to the micro signal. Micro signal setting rules:

1. Micro-signal can't start with a number, but can consist of 6-20 letters, numbers, underscores or minus signs.

2. Micro signal does not support setting Chinese. DOELovEAB, this string of numbers has met the micro-signal setting rules and can be set directly as micro-signal.