Unable to open JNLP files, an error message comes as it is not from trusted developer

Pretty sure that DocuSign has much newer software that doesn't require a Java Runtime Environment to process digital signatures. There is not much you can do about that if the website you are using hasn't upgraded DocuSign. Other than to try to inform them that this tool is so old that many will have problems getting it to work on modern computers. For example, on an Apple Silicon (M1 / M2) Mac, I cannot install Java 1.8.0_381 from java.com. It's too old and not compatible.

I have a similar ancient legacy system that uses JNLP files. Open Finder and navigate to the location where the JNLP file resides. Likely, the Downloads folder. Then right-click (or hold Ctrl key and left-click mouse) and choose Open. This will now pop-up a box where you can read the warning and this time click Open again to allow it to run. The JNLP should then launch using Java. You'll need to do this every time you download one of these JNLP files.

Be careful when overriding the Apple protections for executables like this, make absolutely sure it is legit and trustworthy.