Click the Modify tab, then go to Modify > Extend All.
Click on the string segments that need to be extended.
The object must have another object to intersect with, or the function will not be performed.
Once all segments have been extended, press ESC.
Using this function, the side of the string that is clicked matters.
If the click is clicked on the side that cannot extend due to a No intersection possible warning, the function will not be performed.
In the example shown, the string was only able to be extended toward the surface envelope. There was nowhere for the line to intersect in the other direction.