For SLE 15 UpdateBP run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:plater/SLE_15_UpdateBP/home:plater.repo
zypper refresh
zypper install wkhtmltopdf
For SLE-15-SP5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:plater/SLE-15-SP5/home:plater.repo
zypper refresh
zypper install wkhtmltopdf
For Backports SLE-15-SP6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:plater/Backports_SLE-15-SP6/home:plater.repo
zypper refresh
zypper install wkhtmltopdf