How do I advance a text box without pressing the space bar?

Using plotscripting you can advance the currently displayed textbox with the command advance textbox. It is normally used with suspend box advance, which disables the player's ability to advance textboxes themselves, and resume box advance, which does the opposite.

An example of its use is given below:

suspend box advance
show text box (102)
wait (70) # 70 ticks = approximately 4.5 seconds.
advance textbox #go on the the next textbox, if one has been linked in CUSTOM
wait (70)
show texbox (170) # display the next textbox if it is not linked
wait (70)
advance textbox #remove the last textbox
resume box advance

You don't need the wait(70), advance text box commands after you have shown the last text box of your plotscript if you want the player to advance this last textbox themselves.

Be sure to check and adjust the amount of ticks to wait between each textbox depending on its length. A full textbox would take far more than 70 ticks, an empty one probably less, etc...