Intellectual Property Management - Do I need to sign a Non-Disclosure Agreement (NDA) before submitting my solution proposal?

There is not need to sign an NDA since you are not asked to reveal any confidential information nor provide any solution that is not protected at this point. You just have to provide non-confidential information.

After submitting your proposal, the posting company will evaluate it and directly connect with you.

This is when, if there were an interest, you would request to sign an NDA with the company to keep the conversation ongoing.