Every page has a number. Even if it doesn't include a field which shows its number, it still has one.

However, my recommendation is to not overthink this problem. You can hard code the number and move on. That hard coded number can be used in a field too. eg
{={ Page \* Arabic }-8 \* roman}

The complexity of 'add a bookmark and then use a ref field to find out what page number that bookmark is on' is not always warranted and far from bulletproof. That approach makes sense when setting up a big file where any edits in front of that point could change the start page number and break anything hard coded. However in the case of the TOC this is far less of a problem.
1. It has very little in front of it that could be edited.
2. Edits to the preceding info is unlikely to change pagination because it won't typically be large edits.

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Andrew Lockton
Chrysalis Design, Melbourne Australia