So, as an example I will use the SoapUI forum for this.
Step 1) Gather Data
Open Chrome and press F12, navigate to "Network" and click "Preserve Logs"
Log in to this forum
View the POST request that submits your username and password (for this forum it is "http://forum.soapui.org/ucp.php?mode=login&sid=xxx")
Copy the Request URL
Note the Request Payload
Step 2) SoapUI
Create a SoapUI project (test Suite, case etc etc)
Add an HTTP step
Endpoint: Request URL from above
Click plus sign 3 times for the following:
username username
password password
login Login
Change Media Type to "multipart/form-data" and click "Post QueryString"
Step 3) Store session Data
Right click on the TestCase
Click Options
Click "Maintain HTTP Session"
Viola, you are logged in with Session data. All subsequent actions within the test case will be performed as though you are a logged in user.
Remember:Questions are more important than answers.
The answer is 42 I'm a ninja!