1. Click on the plus symbol next to People in the sidebar on the left to Start chat. 

2. A new window will open where you can find people by manually entering their name or Matrix ID (1). The Matrix ID is composed as follows:
@[UniAccount]:[server-name]
e.g.:  @hf1034:matrix.uni-freiburg.de

Select the person you are looking for from the suggestions (2) and click on Go (3).

If you cannot find a person using the search function, this means that they have not yet signed in to Matrix for the first time. We recommend that you inform your contact about the invitation.

3. A room is created for you and the selected contact. Your contact will be invited to this room. 

You can now send direct messages.