There are various ways this can be done, including screwing right through the frames into the tabletop as you asked about 9with a few tweaks). Probably the easiest solution for you will be to use what are called expansion plates, see image on the right in this previous Answer.
These will be screwed into the A-frames (two screws per) and then into the tabletop (one screw in each). You would often do the last step with the tabletop face-down on the floor (protect it from denting by laying it onto a blanket or similar) so that you don't have to crawl under and try to drive the screws in from underneath, which can be tricky to do properly and is anyway uncomfortable unless you're a contortionist.
You'll need four of these plates. They usually come with their own screws but if not you'll need to buy some. Don't be tempted to use drywall screws, they're a little too brittle to rely on for woodworking applications.
Remember the screws that will go upwards into the tabletop must be short enough not to poke through the top when driven home! You'd be amazed the number of times people have not taken this into account, even experienced woodworkers sometimes forget.
Note: if your top is made from solid wood and not a manmade board material with a veneer you'll need to use the slots that run perpendicular to the grain, not the ones that run parallel to the grain (that would defeat the purpose of using these plates). If your board is plywood or another board material it doesn't matter which slots you use as the top won't expand and contract with changes in humidity the way that solid wood does.