How to do Claude Prefill when you're using OpenRouter in SillyTavern

1. Open the left side menu in ST that has the system prompt and temperature etc.
2. Scroll all the way down to the bottom, where the prompt list is (first entry is World Info, then Persona Description etc.)
3. Click the little plus sign inside a square icon that says "New Prompt" when you hover it.
4. A big box has popped up named Edit.
5. Where it says to enter a name for this prompt, call it "Claude Prefill".
6. Set the Role to "AI Assistant".
7. Set the Position to "Relative".
8. Put your desired prefill into the Prompt box.
9. Click the little floppy disk icon on the bottom right to save this.
10. Now there's a little dropdown in the left menu called Claude Prefill. But it hasn't been added yet. Click the little chain link icon that says "Insert Prompt" when you hover it.
11. Scroll down, now you'll see Claude Prefill is there at the bottom of the list. But it's not turned on yet. Turn it on by clicking the little oval activation slider thing. It'll take a second to react to your click because ST code sucks.
12. You now have your Prefill activated and working! It'll be appended to the end of the context. It'll stay on forever until you turn it off. Click the little pencil icon to edit your Prefill text when you need to change it.