To specify the trajectory from which samples should be taken when an event is captured, you just need to specify the basic trajectory name in the text box (you should not specify any patient-specific prefix). The name is not case sensitive.
The drop down list contains trajectory names which are derived from the marker set file shown at the bottom of the dialogue. You can select one of the names directly, or type the name you want. The text box automatically tries to match what you type, and suggests one of the trajectory names from this list, as you are typing.
The trajectory marker names which appear in the drop down list come from an MKR file. To change the marker set file, click the MKR File button, and browse for your prefered file.