# Specify the source field

To specify the source field as metadata to be injected, perform the following steps:

1. In the **Target injection step key**column, double-click the field for which you want to specify a source field.

   The **Source field**dialog box opens.
2. Select a source field and click **OK**.
3. (Optional) Select **Use constant value** to specify a constant value for the injected metadata through one of the following actions:
   * Manually entering a value.
   * Using an internal variable to set the value: *${Internal.Step.Unique.Count}*, for example.
   * Using a combination of manually specified values and parameter values: *${FILE\_PREFIX}\_${FILE\_DATE}.txt*, for example.