Can the flexfield be filled in automatically?

Summary:

Can the flexfield be filled in automatically instead of user to choose?

Content (please ensure you mask any confidential information):

We define a flexfield and set its value to be queried against the value entered in the previous field. For example, if user select a project number and the flexfield of Fund Source will search according to the project number and list the only value. But we can not make it be filled in automatically and user need to choose this only value manually. Is it possible to make the flexfield be filled in automatically?

Version (include the version you are using, if applicable):