do I need to support them to pick particular technique
As their Scrum Master, you are responsible for helping the Development Team understand the need and benefit for their estimation, and to help them identify a method that supports both their ability to plan and the PO's ability to forecast.
do I need to take a charge and use any technique to estimate the Spike before we onboard into sprint
It is a common practice, and one that I encourage with my teams, to time box spikes in hours/days, and to not relatively estimate them (points). This way, a spike can be treated as a time-boxed event that affects team capacity for the sprint. When the time box expires, then the spike is over, and the team should document and discuss their findings.
Be sure though that the spike doesn't reference deferred refinement, instead supporting analysis and investigation to guide the content and design of future PBI's.