Can I hold a flight seat for my customer for a few hours before I actually issue the ticket?

Yes, with some airlines you can, but not with all of them, and the choice belongs to the airline rather than to you. Many full service carriers allow a booking to be created and held for a set period before payment is taken, and where that is offered your portal can create the booking and show you the deadline by which the ticket has to be issued. Low cost carriers almost never allow it, because their fare is only confirmed once the money is taken, so with those airlines the booking and the ticket happen together in one step.

Where a hold is allowed, the way it works is straightforward. Your agent makes the booking as normal, the portal gives back a reference and a time limit, and nothing is charged yet. If you issue the ticket before that time limit passes, the booking is confirmed at the fare that was held. If the deadline goes by without ticketing, the airline cancels the booking at its own end and the seats go back into sale, so the reference you were holding is no longer of any use to you.

It is fair to be honest about the limits. The hold period is set by the airline, and on a busy route it can be as short as a couple of hours. Some suppliers do not pass the hold facility through their connection at all, even for airlines that offer it when you book with them directly. Extras such as seats and baggage may also not be held until the ticket is issued.

The sensible next step is to ask your supplier which of your airlines support holding a booking, and to keep a reminder against every held reference so that no ticket is lost to a deadline that quietly ran out.