Terry, Gillian is right.
The problem arises when sometimes you see "enter in". How should we understand this?
See definition 2.2 in the Oxford Dictionaries:
2.2 Register as a competitor or participant in a tournament, race, or examination:
Example: They won every race they entered.
Example: The horse was entered in the Martell Cup at Aintree.
You can enter the race, but you can't enter the Cup. So you need a preposition "in".
Some dictionaries give "for" as the preposition. "In" is the correct preposition. Alternatively, you can say, "I am competing for the Martell Cup."