You would need to create a trigger that places a specific lay stake.
Its "Action" should be "lay", and in its "Amount" field you would put the size of the bet. It can be either a fixed number (e.g. 120) or a formula expressing the amount implicitly (e.g. through trigger variables).
And in the "Conditions" of the trigger you might put these:
Any Selection's Back Matched is greater than 0
and Fitting Selection's Lay Price is equal or greater than r_ticks(bm_backp, 2)
where 2 is the number of price ticks.