After selecting Services, Adding a new service, selecting MariaDB cluster (using RepMan) and the latest release, you should reach the Repman configuration page.

Select Replication Manager Database and give your service a recognizable name. Then set the database username and password you want, the default value for username is user, if no password is set, SlapOS will use a random password. It's not possible to give a custom name to database, to avoid possible collision between slave instances.

then scroll down to the bottom of the page and (don't forget) to select the Computer Node where the cluster is deployed.

Click Proceed to continue and request instantiation.

Note, it might take a few minutes for the instance to become available. Refresh the page from time to time until the connection parameter table displays parameteres. Once you see connections parameters such as url, the Database was created and can be used.

Connection parameter Cluster says on which cluster the database was created.
URL is the proxy url for connecting to the database. URL_v4 is is Same as URL but with local partition IPv4, it can be used if the service which use database is hosted in the same machine as the ProxySql.

You can test deployed database with a mysql client.

$ mysql -usoftinst173002 -h2001:67c:1254:3d:bd2:7163:0:4507 -P7032 -p db_softinst_173002
Enter password: 
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MySQL connection id is 4593
Server version: 5.5.30 (ProxySQL)

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MySQL [db__custom_repman_db2]>