There is currently not a way to snap to a 3d point and not get the elevation with this command. The most common use of a linestring is to connect 3d points, so the command has been optimized to minimize the number of clicks to do so.
The command does allow you to edit the elevation using the following steps.
In the coordinate control click on a 3d point. The command will advance to allow you to enter the next point.
Click on the < to the left of "Adding New Segment", this puts you in editing mode for the previous point.
Click on the elevation and make your change. Enter "?" to make the point 2d. This control allows you to use simple math to change the current elevation. For example, enter +5 after the current elevation to add 5.
Click save
Click the middle icon with the green down arrow to return to adding to the current linestring after the last node
If you want a simpler way to create a linestring that does allow you to enter XY and then Z, run the _Quick Line TML macro.
For more details on using linestrings refer to the following document.
Understanding and Using Linestrings.pdf
