What I saw on picture, but it's quite limited info and maybe I am not right - I would think about some structure change.
In the PSP Plannung Massnahmenliste you have
Proj ID and Projectnummer - are they equivalent?
If yes I would NOT use Proj ID in Browse or Edit form at all - use just only Projectnummer - and link all Forms with Projectnummer (e.g. 6-10-000174) - then form linking will be very easy.
Do you use auto increment for the Proj ID?
you can use directly as you placed in the code, but your Proj ID is something like 1,2,3, - and in such case filter with just 1 will not be good - and probably 99% of records will be selected.How can I use the value of the r.attr('data-nu-primary-key') in the select statement of the form "5faebeae9db455f"
If you would have Proj ID as varchar() with id like 5faebeae9db455f then it would be easy.
But if possible I would just stick to Projectnummer.
Maybe you can try completely other solutions with hash cookies for example - but I have limited experience how it behaves between different forms.