Add an assembly reference to a paginated report (Report Builder)
Applies to: 
Microsoft Report Builder (SSRS)
Power BI Report Builder
Report Designer in SQL Server Data Tools
You can embed custom code that contains references to Microsoft .NET Framework classes that aren't in Math or Convert. When you embed the code, you must provide an assembly reference to the report so that the report processor can resolve the names. For more information, see Add code to a report.
Add an assembly reference to a report
In Design view, right-click the design surface outside the border of the report and select Report Properties.
Select References.
In Add or remove assemblies, select Add, and then choose the ellipsis button to browse to the assembly.
In Add or remove classes, select Add, and then enter name of the class and provide an instance name to use within the report.
Note
Specify a class and instance name only for instance-based members. Don't specify static members in the Classes list. For more information, see Custom code and assembly references in expressions in Report Designer.
Select OK.