The fields that can be added in Dashboard are only those that are listed in the available tables in the Uniconta control you are working in.
These fields can be seen in Uniconta by selecting 'other fields' from under the 'Layout' function in the relevant control screen you are working in, in Uniconta. It is not possible to link two tables that are not listed here.
If you wish to create your own link between two tables that are not listed here, then you will need to use an external product such as Power BI or Excel.