The WhereCondition string needs to consist of the field name on the target
form, and a value concatenated into the string.
Something like this:
Dim strWhere As String
strWhere = "[ID] = " & Nz(Forms![Transactions]![Transaction Details
Subform].Form![ID],0)
'Debug.Print strWhere
DoCmd.OpenForm "Entry Form",,, strWhere
For an explanation of the ".Form" bit, see:
http://allenbrowne.com/casu-04.html
The Nz() avoids an error if ID is null (e.g. at a new record.)
If the ID is a Text field (not a Number field), you need extra quotes:
http://allenbrowne.com/casu-17.html

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Allen Browne - Microsoft MVP. Perth, Western Australia
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.
>I have a form called [Transactions] that has a subform [Transaction Details
> Subform]. I would like a form called [Entry Form] to open when I click on
[quoted text clipped - 7 lines]
> DoCmd.OpenForm "Entry Form",,,"[Entry
> Form]![ID]=Forms![Transactions]![Transaction Details Subform]![ID]