How to grab the critical information (entities) which is unknown during the training process?

How to grab the critical information (entities) which is unknown during the training process. (I know how to use general intents and entities to recognize user’s requests.) For example, I want to develop a reminder but I don’t know what user want to remind at design time and the content is arbitrary. The user may say “Remind me to ‘buy a concert ticket tomorrow’” or “Remind me to attend a meeting on Friday”. How to use RASA to grab ‘buy a concert ticket’ or ‘attend a meeting on Friday’?

Can anyone help me please? Sorry for my poor English.