Shingrix has not been evaluated for, and is not recommended for, the prevention of primary infection with varicella virus (chickenpox).
In this case, the patient is not yet immunocompromised and has no evidence of immunity to varicella. The simplest next step is to vaccinate the patient with two doses of varicella vaccine, spaced at least 4 weeks apart, before initiating immunosuppressing medication.
If you wish, you may order a commercial serologic assay to look for evidence of past varicella virus exposure. However, remember that the sensitivity and specificity of these tests vary, and while such commercial tests can detect evidence of past varicella infection, they are not sensitive enough to reliably detect evidence of past vaccination with varicella vaccine.
For patients who lack evidence of past infection or vaccination and who are immunocompromised already, CDC has provided additional detailed clinical considerations here: www.cdc.gov/shingles/vaccination/immunocompromised-adults.html#special-populations.