There are a bunch of ways you could go about this in scriptnode, one approach would be to use a peak node to determine the level of your incoming signal, connect it to a minmax configured as a gate (0-1 in a single step, polarity inverted), then use this to trigger your oscillator.
Have a look at the example below to see this in action. Try adjusting the connected min and max values to set a threshold.
HiseSnippet 1907.3oc4Ys7aaTDFe1jL1woszGzCf3hOzCsnRvqsiis3PRyqhEMol3RE2plt63jQY2YL6tNIFDRf3Hm4Puw+Fbq+If3Lbf+C5cN.eyN65cGm0OpaRoU0QJxyiu462789abKOgE02W3gLV5Q86RQFWF2tOO3vMOjv3nlagLdO7tD+.pWQ0TazuKw2mZiLLl+9xILJr.J7yKVaChCgaQSlBgdrfYQe.ykEjLaq0+BliyNDa5iXto1c00aZI3aJbD8.7LOtDpKw5HxAz8HxsMGF84D+CQFeLdkxUrpZupc4xqznlEoFsQGRmNlUrqVsV8FlUZPpVuFszJHibaayBDdsCHATejwBaHr629PwIbECdLym8TGpbfIpMvY0z6HbrkWQ4rnMOj4X2JVP4iPF3VIhs4Uhsah2kYyFLeh36ZgKTLghzBPi4zg27ZvyLM7JkBdY.IiTPZAEjtNtskGqaPxJR7bIbSNnM6P.8TZnn1Kx343MEvF3AK6RNhtiGLX.A2tVoR2sH7u67Yc5wsBXBdQAeOQ.8g7aemk9tkJrz2uTwgWpSmLWSxFOgiC0KykklFdiivay649Tp2cKdLwoGcvFgqutLEOZYZZUtk5VmZiBdSNK3gco7QYHfhDUv29plaQBHREQzbv95R8BXRHXrE8XvMPoVJf2h5eTfnK3HbFcFXsHr64PBzMgjNZQK.x.M8lT4v8YA8S6HdtYWMsP753Vr.qCyFiykAFAI0EAFi7FuBd6NcnVAI.bA7Ne8EiqWZ1WHl8JuovHogr+CilgKroECml5y7odliId5uOswS6N0wSicxuK9kwgZrwPMmwXn4diKF5aAtiZg4KDiw1L2tNzs4GScfXNgX7FPPlNjdNAwypansqfK5dnfyrRaasOMvicvATuzXOyKz8BBfLyIyby02m5PI9oL9t05OfwoDOPNQmQYg4KcJuL0We.VA2hRuihu8FFc92YBiNNQzh3sOMviX9Fn1KBZkecBs8E8BX7C1k.ttmBE6rWO21P3dKJ.KNm5HK.xXNYoJpwkjikfnMkaGN3egOQKZJGaDsnY7hopxYOZvIBuiB89h9NxHu7tW.y3Ge.baP2ywQbRKgS+3.LgWHpmuzb1HOtzxvepcsovsKKxPGDUpgwH+dthd.SifOT4+iHLGY0Ds64C0jY+PdaXygoGklg6EFmI+NDKPt1uEAR7BtSxp5.0C0aYqApoDnlnmjRWUWFIqJOw3jmJYF.PWofP1JhTa0h3Az.EGKqwaNi4kTHkMQfImjaWAK7sfzyRSkxny.OO5xIqmIfzOfwCJkLzgdZbwnaw765P5uQOvOVEzTaFXbStM8Toi8m.ooScgjYniGAeeWF+wxxB.tJ0fncImNXbUXb6.Z21ruMbrILVd0ygCC9GsujPVQolzNOTJlkOgY4vk04VAXB3iFGuIbBlkTeLq1njoYMH0wQzSTRa0FJWtQiRUWoV4xUpVe05qD0LzNdzuoGXE0OFl4wkCYoNHimEMJYhoFJygMqMBgxkCYYw8kV8HMxGVrXNN9MrNvbj5f6KskeoXDdRLRdv4wsNTlvWaiSRylaZN4bg0WMrYyKVKKXeudABW3BF1lAJUOWHTJev7XvlGpXdjAGTKmk2WLoSxuKKW+7XUPmIDSJa1pVaJYqliqlXHrJI4HH+ybJ6ul9OVtpEwIVFChtTvdQEuMeMfaiEzDWcojiFhsWREiTthVZ0X1oHYBragjBpdDw6.ZfeTi5bpkJ4SnZqos7x6x3tjSSjnxYyiURJz4W30QZrN.AZxgqEJ9gtzVNZ0yJLFP3jD9Zbd5EMWB6G1nwSHd5RmnXLiN6whIt8IYARESefKdRDrj8kNPdp1R1JQqL3X9nO8utUtu7OGJRQxQMTzhKb.VPFIk41ycR4.+eEhjSyBhuXDRQn4uQAw7CAwe8YO62V6rP7VqOUP74E9g+XMU9.4ZI3y4pW6m9m8GM9FGjNmjZ.jf8OshLD5bBRvbiDRKggZtIdxFMldXkJrykSUl4zWlZbrGMpe6uF0yVrw670nNvpY8yw5TksF9lWUp3YoJ0b+3ur4O+I+8ZYC6QjqOcJUMetahoQOb2xI6HqBtReBSvuCO15Bup7oCR4aEeAB69F09D46RkTgvbomPuhA+g19YW8k0+etz9+a69TpsM0NjZ48ROfvBY6iNrdbQr4L4gVojYoJvFZznd0FqVxL5cehdOznC+8wqrbM0AUuR0p0VoTkgr4.euwDP4BErEvwOUazoeCrY8UWdU4IUd0F0ZTs9qDXmXqnZkOnwIzXyaNUdkWIRUrYOuimFu9Lp98UuWpQWlulo57W70obEbSd2dAsYGvSP3jqKXHG1QzOf9CSclNBffiZlLR7fkJQz4+ccJKu9EqMc2rraBLoZ9rtWvtuvuWitl2Ww6UTK.yj09FBwQtjvGEd19Uvec7D1tDKOwShZgVd4VLbFvIjG8pu6JGWzDc7vZPWlM6IVV5G0YHr7rRXkYkvpyJgqLqDVaVIb0Ykv5SlP4uLbTJEowN3w0ZaUsAFayIO0Q0kB5+3PGP9H