Hey @BeckFC, welcome to the Community! :)
As Troy mentioned, there isn’t currently Find Activity search action which would be able to give you the ID for the activity to use in that ID field. That said, we do have a feature request open for a Find Activity action to be added to the Redtail integration, so I’ve added your vote for that. I don’t have an ETA for when that will be added but we’ll contact you by email as soon as it is.
In the meantime, do you have a Zap that creates the activities in Redtail initially? If so, I’m thinking that you could potentially add another step to that Zap to make a record of the activity’s details, including the ID (this info should be output by the Create Activity step), in an app like Google Sheets.
Then in the Zap that’s looking to update the activity, you would add a Lookup Spreadsheet Row Google Sheets search action. This would allow you to search the Google Sheet for the activity and obtain it’s ID. Then in the Update Activity step, you’d select the ID that was found by the Lookup Spreadsheet Row step.
Do you think that approach could work?