If it can be dabbed on to, it should have a mesh on the plastic membrane. They make 2 types. The type with mesh can be dabbed, the type without can't and needs a timber frame building in front of it. If mesh and dabbing you shouldn't dab anything onto the fixing pegs. Although it seems a good idea as they would be the strongest point this is wrong. The whole plastic membrane stops damp and moisture. When drilled to insert a plastic peg the membrane now has a hole. If anywhere now has a weak point and a possibility to fail it would be the peg area. So all adhesive should be put inbetween pegs. As long as the membrane has been installed correctly with correct spacing between fixing (this one looks to be right) then it will hold the weight of adhesive and board no problem