You can't do this with "OpenForm", although you can pass a value in using OpenArgs. If you do that, then when the form opens, you'd just add code like this in the Open event:
Me.YourTextbox = Me.OpenArgs
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I'm not sure what you mean by this:
"I have the MS Access error coding in my code and instead of opening a msgbox to display the error I would like to display my own popup form and populate the label with the error message."
Also, you should really only ask one question per posting, per EE guidelines (unless this is somehow tied in with the other part of your question).