You can think of it like this:
You have two variables, var_speed and direction.
You can represent that movement with trigonometric functions like this:
hspeed=+var_speed*cos(direction), and
vspeed=-var_speed*sin(direction) //Minus here because in Game Maker the Y axis is inverted.
Since sin(a)^2+cos(a)^2=1, the total speed will always be equal to var_speed.

With that said, if you just want simple 45-degree diagonal movement, you just need to set
vspeed=+/-var_speed/sqrt(2), and
hspeed=+/-var_speed/sqrt(2).