Why did Magellan approach the king of Spain?

Answer 1

Magellan proposed to the Spanish king an expedition to find a passage through the New World to this ocean and to sail west to the Moluccas, thus proving that the Spice Islands lay on the Spanish side of the line of demarcation.

Answer 2

The cost of an expedition was too much for a private person to afford, so the support of a royal coffer was required. As a Portuguese man, Magellan first sought the support of the Portuguese monarchs. However, since Magellan wanted to sail westward to find Asia, Portugal was disinterested in this as they had already found a route around Africa. As a result, Magellan petitioned the Spanish monarchs since they were more interested in west-going routes.