Use the Apply button on a role above. It carries the role through to the contact form so we know what you are writing about. You do not need a covering letter and there is no account to create.
Three things carry weight with us. One, something you shipped that ran unattended and stayed correct, with an honest account of where it broke. Two, a live notice picked from the tender list with a note on what you would have flagged in it, in whatever depth you can manage. Three, a bid you watched get technically rejected, and what would have caught it in time.
For the research associate, the third is close to mandatory. For engineering, a repository we can read beats a CV, and a schema you defended beats a framework you listed. For go to market, tell us how you would get a contractor in an industrial estate to show you the process he uses today, given that he has no particular reason to.
If none of the three roles fit, write anyway with what you would build first. We have hired that way before a role existed for it.