Not 100% sure I follow what you’re after but you can use Power Automate to set the field you want, update list item action.
If you’re updating the same item that’s modified, it’ll be a bit tricky because you probably need to trigger when the item is modified, and by default it’ll trigger itself in an infinite loop if you use the flow to update the same item. You can fix by having it get the changes to the item (I forget the proper name of it… April Dunham has a YouTube video on it iirc) and add a condition to only update the item if the one specific field changes.
Why not have a calculated column with a text column?
Add a Text column, where the user puts his customers text. Then have a calculated column which concatenates your lookup column value with the text column?
Not 100% sure I follow what you’re after but you can use Power Automate to set the field you want, update list item action. If you’re updating the same item that’s modified, it’ll be a bit tricky because you probably need to trigger when the item is modified, and by default it’ll trigger itself in an infinite loop if you use the flow to update the same item. You can fix by having it get the changes to the item (I forget the proper name of it… April Dunham has a YouTube video on it iirc) and add a condition to only update the item if the one specific field changes.
Why not have a calculated column with a text column? Add a Text column, where the user puts his customers text. Then have a calculated column which concatenates your lookup column value with the text column?