Hey Valerie,
There is no way to generate a Fills Out Form activity via the SOAP API. One solution is to have boolean custom field which you toggle with every syncLead call you make out of a custom form. This can also be a string if you need to signify separate forms, and you assign a unique value to each form. You would set up a Data Value Changes trigger for this field with the "New Value" constraint being either True for a boolean field, or the name of the form for a string field. In the flow for each of these trigger campaigns, yo'll need a "Change Data Value" Flow step to set this value back to NULL. This way you can trigger based off of each SOAP form fill every time based off of a single parameter. The Data Value Changes trigger will always fire because the field will always be null prior to the form fillout.