Good morning, Deirdre,
That is a frequently question here, and many members are struggle with it.
You can set some settings in Notation Options, but this work one time on two ; in all likelihood, there is currently a bug affecting this in Dorico 5, whatever the version.
I think you will be interested by this thread :
Wild tied note
Until this aleatory bug is fixed, we have not other solution that to use the « Force Duration » input method explained by Mike.