Add an expression to a paginated report (Report Builder)
Applies to: 
Microsoft Report Builder (SSRS)
Power BI Report Builder
Report Designer in SQL Server Data Tools
Expressions are used throughout paginated reports for defining report item properties, filters, groups, sort order, connection strings, and parameter values. Expressions begin with an equal sign (=) and are written in Microsoft Visual Basic. They're evaluated at run time by the report processor, which combines the evaluation result with report layout elements.
Expressions can be simple or complex. Simple expressions refer to a single item in a built-in collection. Complex expressions can contain constants, operators, global collection items, and function calls. For more information, see Expressions (Report Builder).
Note
You can create and modify paginated report definition (.rdl) files in Microsoft Report Builder, Power BI Report Builder, and in Report Designer in SQL Server Data Tools.
Add an expression to a text box
In Design view, select the text box on the design surface to which you want to add an expression.
For a simple expression, enter the display text for the expression in the text box. For example, for the dataset field Sales, enter
[Sales].For a complex expression, right-click the text box, and select Expression. The Expression dialog opens. Enter or interactively create your expression after the '=' in the expression pane, and then select OK.
The expression appears on the design surface as
<<Expr>>.