There are different ways to seal your pot’s drainage hole.
If your pot is not glazed either side of the surfaces (inside or outside), then, you need to waterproof the pot by spraying the inside of the pot with two or three coasts of a clear acrylic sealant for clay pots.
Make sure your pot is clean and dry before you start.
In some cases, it helps to file the drainage hole and around to make the working surface smooth.
Here are some methods:
Method 1: Cover the hole with a piece of thin plastic (milk carton, for example) large enough to extend at least 1” beyond the edge of the hole. Use a bathroom/kitchen silicone sealant to glue the plastic over the hole. When the silicone has dried, fill the hole on the inside of the pot with silicone. Repeat this to also cover the outside of the hole. Let cure for 24 hours.
Method 2: Insert a cork into the drainage hole. Adjust the size of the cork by shaving with a knife and put plumber’s putty to fill any gaps around. The cork and putty make a water tight seal but are easily removable if you need the pot for planting later. Notes: Be sure to thoroughly test the seal to make sure that there is no leakage. 48 hours is usually safe.
Note that Method 1 is more secure than Method 2. Both methods are subject to failure, so be prepared for a possible leak.