The two steps that you must do in Salesforce to map a Marketing Cloud Account Engagement prospect custom field to a Salesforce field are:
Add a new custom contact field in Salesforce. A custom contact field is a field that you can create and add to the contact object in Salesforce, which stores the information and activities of an individual person who is associated with an account. You can use custom contact fields to store data that is specific to your business needs, such as industry, product interest, or satisfaction score. You can map a Marketing Cloud Account Engagement prospect custom field to a Salesforce custom contact field, so that the data is synced between Marketing Cloud Account Engagement and Salesforce. To add a new custom contact field in Salesforce, you need to go to Setup > Object Manager > Contact > Fields & Relationships > New, and follow the steps to create the field.
Add a new custom lead field in Salesforce. A custom lead field is a field that you can create and add to the lead object in Salesforce, which stores the information and activities of a potential customer who is not yet qualified. You can use custom lead fields to store data that is specific to your business needs, such as lead source, lead score, or lead status. You can map a Marketing Cloud Account Engagement prospect custom field to a Salesforce custom lead field, so that the data is synced between Marketing Cloud Account Engagement and Salesforce. To add a new custom lead field in Salesforce, you need to go to Setup > Object Manager > Lead > Fields & Relationships > New, and follow the steps to create the field.