The secret lies in workflow variables.
But first, create a Multiple lines of text field. I called mine "Actions Log", because I'm logging the workflow actions. Then you'll need to use either an existing workflow or - if append text is the only action you need to take - create a new workflow.
Within the workflow, you're going to need two variables. One to hold any existing text in your Multiple lines of text field and one to hold a copy of the original text plus any appended text.
|To create a Workflow variable, you need to first select "Set Workflow Variable" from the Actions drop-down.|
|Having selected "Set Workflow Variable" you're now prompted to create a new variable.|
|Naming the new variable and setting the content type are both done in the same dialogue box.|
|This step causes the value in the Action Log text column to be copied into the ActionsLog variable.|
|Create a second variable called ActionsLogAppend which will hold the original Action Log text and any new appended text.|
|Click on the ellipsis to launch the String Builder window, which in turn creates the ActionLogAppend value.|
20/11/2019: 10.55 - 18 months to End of Lease acknowledged by John Smith:
[Content of first variable]
So in the String-Builder, add a lookup for the Date (and the Time, if you want it), and add the identity of the person who carried out the action (that'll be the value for ModifiedBy).
|I set the Date and Time to "Short Date" and "Short Time" content types, but you may prefer a different setting.|
|This is what the string looks like in the String Builder window. I put a line return after the ModifiedBy call for clarity, putting the appended text on a new line ...|
[ModifiedDate]: [ModifiedTime] - 18 months to End of Lease acknowledged by [ModifiedBy]:
But if you wanted to, you could copy this combined text back into the Multiple Lines of Text field via the workflow.
That's it ... I hope this helps someone.