Can I use this for a new install of a undermount sink?

Thank you again for the question. It is possible the Loctite Power Grab Ultimate White might work but it is not recommended for safety reasons. A construction adhesive should never be the sole source of support for under mount sink installs. The successful use will depend upon several variables such as the composition and condition of the counter surface underneath, the composition of the sink itself. Under mount sink clips or other mechanical acceptable support systems should also be used for safety reasons if using a construction adhesive.