The "Subscribe to newsletter" / "Apply discount code" / "Send chat
message" pattern. Two action variants:
- text (default): pill button with label like "Subscribe"
- icon: 44×44 square icon button (chat send arrow / search apply)
Distinct from add_form_field_v0 (label-above, no inline button)
and add_search_bar_v0 (no trailing action button). Optional
leading_icon adds an icon inside the input itself. Plus the v1
corpus prompt landing-newsletter-signup (25 v1 prompts total).
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