A vague or generic call to action wastes everything the rest of the letter built up; the final ask needs to be as deliberate as the headline that opened it.
Pair an Imperative Verb With a Specific Benefit
"Start My Free Trial" outperforms "Submit" because it restates the outcome, not just the mechanical action.
Make the Next Step Unmistakably Clear
The reader shouldn't have to guess what happens after they click; state it plainly if it isn't already obvious from context.
Repeat the Call to Action More Than Once
In a long letter, offering the chance to act at multiple points accommodates readers who are ready to decide before reaching the very end.
Next step: Use the Sales Letter Writer to draft several call-to-action variations and compare which reads strongest.