Of 'yapmak'. 'Bunu senin yapman' is relative clause here.
Lazım(It is necessary.)
Ne lazım?(What is necessary?)
Bunu (senin) yapman(That you do this.)
So 'Bunu senin yapman' is the subject in the form of a relative clause and 'lazım' is the predicate despite not being a verb.
For a better understanding you can analyze the relative clause. For example we can turn it into a finite indicative sentence:
Bunu sen yaptın: You did this.