That’s not something you can do at all or what is shown in your BP-Screenshot. I do believe that the template parameter has to be known at compile time.
The equivalent to your screenshot is simply:
FName LocalRowName = FDataTableRowHandle.RowName;
FYourStruct* OutRow = FDataTableRowHandle.DataTable->FindRow<FYourStruct>(LocalRowName, "");
If you want to see how GetDataTableRow is implemented see K2Node_GetDataTableRow.cpp.