That depends on RouterOs version.
The way for v7.x is explained in this thread here:
Primary gateway with static ip address not activating
extremely simplified:
Primary gateway with static ip address not activating - #23 by jaclaz
Only as side note, if you have n ISP's, you actually need n-1 recursive routes, as when internet is not reachable on ALL your n-x routes and you reach the nth one, it's game over, no further possibilities left.