Linking a project to a program through a workflow

Hi, I am looking to link a project to a program as we have enhanced permissions preventing users from linking to a program when creating a project. 

I created a custom field that references the programs. However I could not find a way to make a workflow that takes the value of the custom field and allows the project to be linked to the program. I know there is a project to portfolio link, but how do we link a project to a program? 

Thank you,

Saravanan