Manual adding of observations is only possible for Traditional projects, not collection projects. So you would need to use one of those - have you tried a traditional project?
I created a traditional project.
I can add observation on demand - this is resolved
New problem: I cannot specify a required time window (April 2024). https://www.inaturalist.org/projects/___test_1
Yes, you are right. First I thought that’s really strange, bur since you add the observations manually anyway, just choose the ones within the time frame. And make it very clear to the projects members, maybe by adding “only April 2024” or something similar to the title. I don’t know if you as admin could delete observations which don’t meet the requirements.