Yes, you can. The line in your sources needs to be like this:
deb http://foremanhost/pulp/deb/Default_Organization/Library/custom/ubuntu_18_04/ubuntu_18_04_main default all
Pulp3 is only configured to publish the repo under default release and all component. I think it will get addressed in a later version to also publish it so that you could use bionic main etc.
I second the answer by @rgp, but want to add one more thing:
You say your /etc/apt/sources.list entry is:
deb http://foremanhost/pulp/deb/Default_Organization/Library/custom/ubuntu_18_04/ubuntu_18_04_main/dists/default/ bionic main
I think it should be:
deb http://foremanhost/pulp/deb/Default_Organization/Library/custom/ubuntu_18_04/ubuntu_18_04_main/ default all
That is, not only is there the default all vs bionic main issue already mentioned, but also your URL should point at the repo root and not a subfolder of the dists/ folder.