Davis,Here is a way:
(Assuming that there are no time signature changes during the music)
1. Switch to Time Signature Tool.
2. Double-click the first measure.
Set the Time Signature to one beat.
Set the region to "Measure 1 Through End Of Piece".
Check the "Rebar Music" checkbox.
Click OK.
3. Double-click the second measure.
Set the Time Signature back to the original time signature.
Set the region to "Measure 2 Through End Of Piece".
The "Rebar Music" checkbox should still be checked.
Click OK.
Now everything should be OK, except that the time signature for the first measure is displayed as one beat.
4. Double-click the first measure.
Click the button "More Choices", and check "Use a Different Time Signature for Display".
Set the "Time Signature for Display" to the original time signature.
Set the region to "Measure 1 Through 1".
Click OK.
After Music Spacing and Update Layout, you might find that the first measure (the pickup measure) is too wide.
In that case, go to
Options menu > Document Settings > Music Spacing Options...
and set the Minimum Measure Width to a lower value. Click OK, and try another Music Spacing and Update Layout.
Peter