Please refer How to connect to Datamart using BI Connector? and proceed with merging tables.
On successful connection, Click on the Database name and connect to the required database.
list of Subject Areas will be displayed. Select a Subject Area from the list.
Select one or more tables from the list and click on Edit button.
Click on 'Merge Queries'. Select table from the drop down list that needs to be merged.
Select the column GZID on both tables and select the 'Join Kind' to Left Outer and click Ok button.
Note: GZID is a BI connector virtual column that is used to ensure that PowerBI uses OBIEE joins.
Please make sure to select the fact table on the top and the dimension table on the bottom while merging the tables.
Select the required columns from the merged table for visualization.
Click 'Close and Apply' option to apply changes.
Now self created Datamarts are ready to be used in Power BI Service.