The difference between these two prepositions comes from the purpose with which you use them.
When you want to express that something is on top of, upon, or fully aware of and informed about, you should use onto.
However, if you are using a verb phrase of which on is a part of the verb phrase, you should separate the words.
A common verb phrase that has on is: log on or catch on.
When looking at your example above, it looks like the verb is come. The verb phrase does not include the preposition on.
For this reason, you should use the combined formed of the verb, onto. The correct form of the sentence you have above should be “Come onto the property.”
