How do I store keys/values in JOSM?

I have been adding some postboxes lately and I like how JOSM learns the keys and values that I enter. e.g. post_box:type=pillar,lamp,etc., royal_cypher=VR, EIIR, etc,. After entering a few, it is easy to pick the right tag from the drop down list. However, next time I start up JOSM, it has forgotten all the tags that I entered last time and I need to start again with the less common ones. Can I make JOSM store these values?