ENGA (engaged) is a FAMily tag only. So you will need to join the engaged couple as a family then add the ENGA tag.
I would probably also add a _NMR (not married) tag, and then a _SEPR (separated) tag.
Alternatively, as this seems a lot of work for an engagement that didn’t survive, I would be tempted to ignore it completely, or just add it as a note (perhaps a shared note so it can be attached to all three people involved).