Cell Value to Trigger Popup Message

[ShakesPURR]

Hi there!
I am a new member and need help please.
How do I trigger a popup message from a specific cell value in excel?
(I am using Office 2013.)
Please help and thank you!
H

 

[ghrain22]

select the cell, go to Data in your ribbon and select data validation. There you can create some settings and an error alert.

 

[ShakesPURR]

I created a form with content control.
The last thing I am trying to get right is:
When a person completes the form and ticks the box for "Proof not attached", it automatically creating value "2" in cell D23.
I would like a popup to then be triggered saying "Not allowed" or something along those lines.
And for the life of me I cannot get it right