You cannot paste plain text directly into the scripts workspace.
Copy script steps in the scripts workspace and then paste them as new steps in the same script or as steps added to a different script.
Copy text--such as a calculation that you can then paste into a dialog opened from the scripts workspace.
You cannot copy/paste the following script step directly into the scripts workspace:
Set Field [Table::Field ; Table::Field + 5]
But if you open the scripts workspace and type in "SF" to add the set field step, you can then click the gear icon, open the dialog for the calculated result and paste "Table::Field + 5" into it as it's calculated result.
Not at all ideal, but it's what you can do here.