President Reagan gave a local television station's Job Fair a boost last night with a telephone call from the White House, urging firms to "make quite a dent in unemployment" by hiring just one worker.

"Call now," the president said, "you can make this 'Job Fair' work." Echoing the appeal he made before Christmas, Reagan said, "if those businesses that can, if they would just take a look and see, they could take on another employe and they could make quite a dent and speed the recovery just that much."

The station, WDCA Channel 20, had used a videotaped appeal from Reagan to open the two-hour show.

As part of the event, job seekers lined up to fill out applications and to take their turn before Channel 20's television cameras at Wheaton Plaza in Montgomery County to make personal appeals for jobs. Meanwhile, 40 job counselors staffed telephones and took applications from viewers.

WDCA officials estimated that nearly 2,000 jobs had been registered with them by last night at three locations--Waterside Mall in the District, the International Trade Center in Alexandria and Wheaton Plaza, although cameras were set up only at the last site. They said a complete tally wasn't available.

The event is a joint project of Channel 20 and state employment offices in D.C., Maryland and Virginia. Over the last six weeks, job placement counselors were trained to take applications and answer telephone inquiries.

Yesterday, the counselors spent more than 12 hours talking with potential workers. "Some of the people are looking for better jobs, but most are unemployed," said James F. Parkins, director of the Wheaton office of the Maryland Employment Security Administration.

Taft Broadcasting Company, which owns Channel 20, broadcast similar programs last night in four other cities: Cincinnati, Kansas City, Birmingham and Buffalo. Job applicants and job openings will be matched by computer, and in about 10 days local job placement counselors will be notifying applicants about possible employment.