Trans World Airlines announced an innovative marketing proposal yesterday to induce travel agents to help make their new routes to Florida a success.

TWA said it would offer a discount of $10 on any round-trip TWA ticket to Florida, even discounted promotional fares, that are written by travel agents. And on top of the normal commissions that travel agents get from the airlines for selling their tickets, TWA will give the agent an additional $10 handling charge for writing a Florida ticket on TWA.

The plan is subject to the approval of the Civial Aeronautics Board, which is likely to be receptive.

"The offer is being made exclusively through the travel agent because we recognize that the travel agent is the key to our being successful in the building of business in this new market for TWA," Donald M. Casey, TWA senior vice president for marketing, said. "That's why we are offering this opportunity to the travel agent to give his customer a better deal than even we can offer."

It is likely that this TWA plan will spur other airline proposals. Competition into Florida will be keen this winter since deregulation and the CAB have allowed airlines to begin new routes easily. Both TWA and United Airlines, which have had extremely limited service into Florida until now, are moving into the state in a big way this winter, as are several of the smaller airlines.

TWA will begin services Dec. 15 with 38 new daily nonstop flights between 10 Northeast and Ohio cities into Florida. Washington is among the new cities being served. It has had a half dozen flights daily until now.

In another development yesterday, shareholders of Trans World Corp., TWA's parent company, and Centruy 21 Real Estate Corp. approved a merger agreement whereby Century 21 will become a wholly owned subsidiary of Trans World Corp.