Method: identify the exact context your input lands in, then craft the payload that breaks OUT of that context. Prove execution by making your injected JavaScript call /m03/api/solve/r_js (e.g. with fetch()). No external collector needed. Server-rendered sinks also auto-mark once a value that would run is reflected/stored here.
Your input is dropped straight into a <script> as the contents of a single-quoted string. HTML-style tag payloads are inert here - think about the JavaScript syntax you need to end the string and start running statements.