Identifiers for keywords

I know that EML doesn’t include identifiers for keywords, although it can be done through annotations. What is the preferred syntax for including identifiers for keywords? For example, ENVO recommends something like plant matter [ENVO:01001121] for MixS.

But when I use keywords from GCMD, for example, should I use the same bracketing convention for identifiers? Or does anyone have strong feelings that it should be recorded another way?