create a file /etc/sbopkg/repos.d/80-slackbuilds-master.repo with this content (one line)

slackbuilds master "Official git master from slackbuilds.org" custom git git://slackbuilds.org/slackbuilds.git@master ""

then, to have it selected as the default repository, edit the lines of /etc/sbopkg/sbopkg.conf with the variables REPO_BRANCH and REPO_NAME

REPO_BRANCH=${REPO_BRANCH:-master}
REPO_NAME=${REPO_NAME:-slackbuilds}

and do a sbopkg -r (to sync).

For temporary usage

sbopkg -V slackbuilds/master -r

sbopkg -V slackbuilds/master -i my_package_or_queue

would do.