Hi there and welcome
You do the following
First CLOSE iMovieGo to the finder and locate i Movie and click “Show Package Contents”
Navigate to Contents/Resources/English.lproj and do a “Get Info” on TrailerStrings.strings. Add Read/Write to your user to the permissions list. You will also need to add READ WRITE permissions for the Contents folder, the Resources folder and the English (or whatever language you are using) folder.Add Opens with TextEditEdit the Credits Line being sure to leave the underbar “_” thereYou can see the process with screenshots here
I can also do these steps for you, and have made a request via Additional Services. This is optional, and if you choose to do this, I can, with your permission, remote into your computer while you watch and do all these steps (and deal with any problems that arise during them) for you. Or if you do not choose to take this service, we can continue here. This is no problem at all. Just let me know please.
Else let me know how you go please.