I use Node Red for this. Below is a flow that you can import with HTTP endponits. They control an “input boolean” which is created in Home Assistant. The input boolean is the Smarrthings equivalent of a virtual switch. You can make the input boolean by just going to configuration, helpers, add helper, and then select “toggle”. Type a name and then that entity will be controllable in Node Red.
[{"id":"fd7b6fd07baabc93","type":"http in","z":"115a2d09.b31883","name":"Turn On","url":"/endpoint/iftttturnon87889","method":"post","upload":false,"swaggerDoc":"","x":120,"y":2780,"wires":[["90948479876a50fa","01de9208fd326aa7"]]},{"id":"90948479876a50fa","type":"template","z":"115a2d09.b31883","name":"page","field":"payload","fieldType":"msg","format":"handlebars","syntax":"mustache","template":"<html>\n <head></head>\n <body>\n <h1>Connection Message Received</h1>\n </body>\n</html>","x":330,"y":2860,"wires":[["48d478416c0b42ab"]]},{"id":"48d478416c0b42ab","type":"http response","z":"115a2d09.b31883","name":"","statusCode":"","headers":{},"x":510,"y":2860,"wires":[]},{"id":"01de9208fd326aa7","type":"api-call-service","z":"115a2d09.b31883","name":"Turn on Boolean","server":"ae531ce.a39906","version":3,"debugenabled":false,"service_domain":"input_boolean","service":"turn_on","entityId":"input_boolean.something","data":"","dataType":"jsonata","mergecontext":"","mustacheAltTags":false,"outputProperties":[],"queue":"none","x":420,"y":2780,"wires":[[]]},{"id":"b01afc467ed97250","type":"api-call-service","z":"115a2d09.b31883","name":"Turn off Boolean","server":"ae531ce.a39906","version":3,"debugenabled":false,"service_domain":"input_boolean","service":"turn_off","entityId":"input_boolean.something","data":"","dataType":"jsonata","mergecontext":"","mustacheAltTags":false,"outputProperties":[],"queue":"none","x":430,"y":2920,"wires":[[]]},{"id":"bb5e2713fc622e1a","type":"http in","z":"115a2d09.b31883","name":"Turn Off","url":"/endpoint/iftttleave788736","method":"post","upload":false,"swaggerDoc":"","x":110,"y":2920,"wires":[["90948479876a50fa","b01afc467ed97250"]]},{"id":"ae531ce.a39906","type":"server","name":"Home Assistant","version":1,"legacy":false,"addon":false,"rejectUnauthorizedCerts":true,"ha_boolean":"y|yes|true|on|home|open","connectionDelay":true,"cacheJson":true}]
Then on the IFTTT side you setup two actions to send the webhooks to Node Red