Hi Oussama,
The excel file explains what the issue is - based on your description, I had thought you were using the pop-up of a layer's web map (such as you would use in Collector or Explorer).
If you are creating a link within Survey123, you would need to use ${testdata} (as opposed to {testdata}), which you have done; however, the placeholder needs to be by itself in a concat statement, and you would need to join pieces together:
concat('<a href="arcgis-survey123://?itemID=728ae32766d24ad8b664a24505998e77&field:imsma_id=', ${testdata}, '">237 Hazard Report</a>')