I got it to work - I'd made a mistake by using the .value property of the textbox instead .text. In the end, I was able to enter info into the text box, hit the enter key - and have the vba use the text from that tb, as I'd hoped. Simple now that I see it.
Private Sub tbKeyWordsLike_KeyDown(KeyCode As Integer, Shift As Integer)
If vbKeyReturn = KeyCode Then
cbLookup_Click
End If
End Sub
followed by:
Private Sub cbLookup_Click()
mywords = Me!tbKeyWordsLike.Text
{misc code using text from "mywords" here}
End Sub