In case you want to add a field with a custom name to a node in a session right click on the node name in the tree structure and select Actor Key (in the example of an actor node). The new field appears as Keys.Key.Standard Template Key in the list of the field names. In order to change the name of this new field right click on it, choose Open in External Editor and right-click the Change key name button and then OK. After you have changed the name, click on Apply for the name to change. Note that the new name will always start with Keys.Key., which is there by default. Only key fields can have their name changed. Note that you can add a field with a custom name (key option) only in Session, Content, Actor, Media File, Written Resource and Source.
After you have added and customized the new key field(s) and you wish to use it other nodes, you can create a favourite for this node so that you can re-use it instead of repeating the whole process for every new node.