You can set your service to 'TheCenter' map by doing the following steps -
1. Navigate to the Commandline Manager in TCAdmin and open it.
2. Select the Custom Commandlines tab and create a new commandline (you don't need to create a new commandline if you have already created one).
3. You should then see a list of parameters that you can set, locate the 'Map ID' parameter and tick the checkbox next to it, then change it from TheIsland to TheCenter.
4. Simply scroll down to the Save button and then save it, once you have done that simply click on 'Select' and then start your server.