That feature is not included in your version of Pi-Hole, but as you have done, any DNS server can be listed in the custom area.
The beta of FTLDNS has it and I believe I read that it will be included in the V4 release.
Pi-hole version is v3.3-181-ga7e7680 (Latest: v3.3.1)
AdminLTE version is v3.2.1-195-g4355bde2 (Latest: v3.3)
FTL version is vDev-5ecab0a (Latest: v3.0)
The beta FTLDNS page is here: Help Us Beta Test FTLDNS