Requester needs to create tickets under different sites

Requester needs to create tickets under different sites

We have 3 sites built under our MSP Account. The default site and 2 other locations where our customers' equipment is located.
Our support staff are set-up as requesters under the MSP account/default site via an AD connection.
How can we set the system up so that our requesters can open tickets for all 3 sites?
Currently when they log in they only have access to the default site in the Site drop-down, but obviously our customers are calling in with issues from all 3 sites.

Thanks!

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