An open goal shot, but if your startup has already collected launching customers or letters of intent, you have an edge with many investors. There is almost no better proof that your product or service works.
Are there no launching customers or letters of intent yet? Then start looking for your early vangelists, such as companies willing to buy your product before it is fully ready for the market. Sometimes they are also willing to invest in the further development of your product. Do you have a clear idea of the problem you are solving for this group?
Whatever variant applies to you, what matters is market validation: can you show that there are people who (want to) buy your product or service.