You can just use the random function and floor it, but if you want to make random shapes, that would be more difficult. Adding random tiles to random locations is easy though.
For example:
tilemap > set tile at
X: [int(random(tilemap.width)/{put tile dimensions here})] to {whatever tile}
Y: same as above but using tilemap.height
To remove tiles you just do the same thing, but with the 'erase' action.
To make certain shapes you would have to make a whole bunch of rules for where tiles can go, and then use repeat loops or something similar to check each tile in an area for those rules, or something like that.
...Or ask someone who knows more than I do about random generation.