SSIS set variable at runtime

You can add a Script task. Add your SSIS variable(s) to the ReadWriteVariables property in the Script Task Editor.

You can reference the variable(s) in your script using the following format: Dts.Variables["MyCaseSensitiveVariableName"].Value

Lots of ways really, but the one I use most frequently is to use an execute SQL task and set the result set to single row and then put the results set into the variable.

Use Expression Task instead. In the "Expressions" pane, put the variable to set on left, then equates it to the intended value.

@[User::VariableToSet] = some expressions...