There a several ways to communicate with acrobat from vba/vbs (see Acrobat SDK / IAC section). But for me the best is to work with Acrobat Form API, which allows to work more ore less direct with js-code.
Following you will find an vbs/vba example how to add text on the first page (zero based: first page = 0).
Please look for further methods and properties and especially for defining the rect (place/array to set the box) at the "Acrobat JavaScript API Reference".
Good luck, Reinhard
Path = "D:\Test.pdf"
Set App = CreateObject("Acroexch.app")
app.show
Set AVDoc = CreateObject("AcroExch.AVDoc")
Set AForm = CreateObject("AFormAut.App") 'from AFormAPI
If AVDoc.Open(Path,"") Then
'// write some js code on a vbs variable
js = "f = this.addField(""newField"", ""text"", 0, [250,650,20,600]);" &vblf _
& "f.value = ""any Text""; " &vblf _
& "f.flatten"
'//execute the js code
AForm.Fields.ExecuteThisJavaScript js
end if
Set AForm = Nothing
Set AVDoc = Nothing
Set APP = Nothing