Discussion on:

Office challenge: How to use input values as default values in an Access form

Actually, I can think of three different ways to do what you ask, but one way in particular probably answers your question most directly. FIRST, set up public variables for the "defaults" in question and initialize them (to blank, zero, etc.). SECOND, create After_Update event triggers such that the variables are populated as data is entered for each related field (this will also allow you to change the "default" by overtyping with new information). THIRD, create event triggers such that variable values are entered in the related fields when a new record is begun. Certain precautions could be taken in this last set of triggers to minimize the chance of partially completed records.

You can use the following Code in the form After update event:
Field1.DefaultValue = """" + Field1.Value + """"

field2.DefaultValue = """" + field2.Value + """"

field3.DefaultValue = """" + field2.Value + """"

This will replace the default values by the latest values entered on the form at each record insert.

It depends on how the data is organized.
For the example, one would expect the data to go in a least two separate tables: one for Salesone ID information, another for order information. In which case the answer is trivial.
The challenge implies that the intent is to repeat the Salesone ID in every record in a single table, hardly good relational practice.

This is close to LKJCPA recommentation.
Do an afterupdate event on the field in question setting the default value in the properties for the current new default value entered in form. It would stay the same value if nothing changes.

Quick and dirty code:
Private Sub Field1_AfterUpdate()
Dim txtDefault As String
txtDefault = Me.Field1.Value
Me.Field1.DefaultValue = Chr(34) & txtDefault & Chr(34)
End Sub

I would create an unbound form, set the Tag property of each text box to the field name of the table to which the information is being added. Place a "Save" button on the form that when clicked would scrape the values - via VBA code - from the text boxes and save them to the corresponding fields in the target table. The user-entered information will remain in the form's text boxes until the user manually changes/clears them or until the form is closed.

It's easy to bind a table to a drop down list. Run a macro (or, if you really must, some soon-to-be-needing-to-update VBA) to detect an new entry and add it to the table, or, if your skills aren't quite up to that, offer an option to add a new entry and open a sub-form.