In order for the system to automatically confirm that you control the private key of the API DASH address you must do two transactions:
1: send some DASH (any amount you like) from your primary DASH address to your API DASH address (use coin control to ensure that your primary DASH address is the source of the funds).

2: send the DASH from step one (use coin control), which are now on your API DASH address to the CrowdNode deposit address.

The amount you deposit in step 2 will be credited to your CrowdNode balance just as a regular deposit from your primary DASH address, but the system will automatically see that it came via your API DASH address – and this confirms that you control the API DASH address.